The titular Ring in The Lord of the Rings is one of the most complex and interesting magical items in film or literature. The way that it affects the nature of its owner is fascinating, and it's equally intriguing to see how it affects everyone else around it as well. The Ring corrupts, and though there are some who can handle and resist its power better than others, it always seems to fundamentally change anyone who touches it in the end. Frodo and Gollum are two characters who come under the influence of the Ring, with Frodo being resistant at first and slowly falling to the Ring's will, and Gollum being an extreme example of what could happen if one were to be in possession of the Ring for too long.
That being said, there are a few characters who hold onto the Ring for a period of time and yet are able to give it up freely, namely Bilbo and Sam. Because Bilbo has been in possession of the Ring for so many years, it seems strange that he is able to give it up as easily as he does at the beginning of the story. Sam's case is interesting because it doesn't seem as if the Ring really affects him at all, despite being near it for a large portion of the story, and actually holding onto it at one point (though to be fair, it was for a very short amount of time). What is it about these two Hobbits in particular that makes them so resilient against the effects of The One Ring?
An argument for Sam's ability to give up the Ring is that he only held onto it for a short time. However, Gollum was almost instantly corrupted by the Ring when he initially came upon it. He was so overwhelmed by it that he killed Deagol in order to protect the Ring and keep it for himself. Additionally, Bilbo hung onto the Ring for quite a long time before he eventually gave it up, which also refutes the theory that the Ring's corruption lies within a certain time period. There is an argument to be made that for both Hobbits, their innate goodness was what kept them from being completely overtaken by the Ring's power, but is that all?
It may have something to do with the intent of the Ring-bearer, and what their ultimate goal with the Ring is. Bilbo was in possession of the Ring for many years but only gave it up near the end of his personal journey, when he would not be in need of it. It also probably helped that he didn't know the full extent of the Ring's power or significance, and was guided into letting go of it by Gandalf, whom he trusts fully. Perhaps if Gandalf had asked him to get rid of the Ring when he was younger and still actively "in need" of the Ring, he would have had a much harder time giving it up.
It is, of course, important to keep in mind that The One Ring isn't a purely inanimate object. It corrupts, yes, but it also has the ability to shift things to its will. In The Fellowship of the Ring (the book) Gandalf tells Frodo, "[Bilbo] needed all my help, too. And even so he would never have just forsaken it, or cast it aside. It was not Gollum, Frodo, but the Ring itself that decided things. The Ring left him." The Ring had decided that it had fulfilled its purpose with Bilbo, and had already chosen to leave, likely to try and make its way back to Sauron, or something along those lines. All Gandalf needed to do was give the suggestion to Bilbo that he should give up the Ring, and he was able to do it because the Ring had already made the decision.
As for Sam, his resistance likely has to do with his innate goodness and loyalty. His love for Frodo helped him to overcome the pull of the Ring and his inner pureness was never fully pierced by the Ring. In the books, it also states that "he knew in the core of his heart that he was not large enough to bear such a burden...The one small garden of a free gardener was all his need and due, not a garden swollen to a realm; his own hands to use, not the hands of others to command." He knew that great power was not his fate and he didn't crave it, so he was not as easily corrupted by the Ring. If there's any credence to the theory that the Ring has a hand in choosing its bearer, there's also a chance that the Ring knew this about Sam, and never tried to exact a strong hold on him because of it.
In terms of innate goodness, it's also interesting to note that every other Ring-bearer (save for Bilbo, Frodo, and Sam) had to kill the previous owner of the Ring in order to come into possession of it. Perhaps the manner in which the Ring comes into one's possession is indicative of how it will affect the bearer. Bilbo and Sam both came into possession of the Ring by taking it in a relatively peaceful manner. Sam only held onto it while he thought Frodo was dead, and Bilbo simply stumbled upon it, and kept it by outwitting Gollum. In fact, Bilbo's possession of the Ring in particular was characterized by pity, as he encountered Gollum and saw what he had become. The compassion he shows to Gollum by not killing him could have been a large factor in why the Ring did not corrupt him the way it did other Ring-bearers.
One of the things that is so fascinating about Tolkien's work is that there are so many questions in The Lord of the Rings that are never fully answered, despite his vast and intricate worldbuilding. This leaves the fans with the ability to read deeper into the texts or even the movies and try and figure out some of these complications for themselves. While there is no official answer for why Bilbo and Sam were able to be rid of the Ring without too much issue, there is a lot of supporting evidence that suggests it was partially due to their inner morality, as well as partially because of the Ring choosing its own path. Either way, it's an interesting thought experiment and proves once again just what special creatures the Hobbits really are.