system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] So what does the One Ring *do*?

Forgive me, I haven't read the books for years but I recall that Sauron forged it to give himself power over users of the other rings and that it made him a formidable warrior (though was he formidable without it?) It was known to cause invisibility by the age of The Hobbit and, by the time of LOTR, had some ephemeral link to the intangible power of Sauron and his minions. But what did the ring DO? In the discussions of what to do with it, some proposed using it -- and that was Boromir's ostensible motivation to take it from Frodo -- but what could a wielder of the One Ring do? What benefit would have been derived from mastering the ring and using it against Mordor?
gpt
So the simple answer is that The One Ring does exactly as it says on the box (or in this case inscribed in the ring) "One ring to rule them all, One ring to find them, One ring to bring them all and in the darkness bind them." In other words it dominates others, particularly the other Rings of Power.

However, as most things in Tolkien's works, it's not all that simple. Magic and power in Tolkien's world is oftentimes misunderstood. You first have to understand that Sauron didn't create the Ring to grant him some supernatural ability. Rather he created it to enhance something he already possessed, the ability and desire to dominate and control. In order to do this, he somehow literally poured his spirit into it, and as such the Ring is ment for him and him alone. In doing this, he became more potent, but not without risk, the Ring could be destroyed, and with it Sauron himself, or someone of great power could wrest control over the Ring and claim Sauron's power for themselves.

So now we come to the meat and bones of your question, what does it do? For Suaron, it would heighten his ability to dominate and control others. With the others who had Rings of Power, he would be able to see their thoughts, and bend their wills to an extent. They would not necessarily be mindless automatons, but neither would their will and desires be their own. The masses would ultimately bend the knee and their will to Sauron, being ruled over by him not just as a king rules over his kingdom, but in a more direct, internal way.

Now what about others who would want to use the Ring? Well in a way it would do the same. We only have to look at the vision Sam had when he wore it for a brief time. He saw that the Ring would make him a mighty Hobbit, and he would use the power to create a vast world wide garden, and all others would join him in creating and caring for it. It is subtle, but it is still a vision of control, Sam, who truly does desire to have a simple life of a gardener, would see all peoples share that desire and carry it out. For someone like Boromir, he had visions of using the Ring to inspire and rally his people into being a victorious army to protect his realm.

But the problem is that the Ring is not for them and as such they could not unleash it's full power because they are not as strong as the one who made it for himself. Galadriel counciled Frodo " 'You have not tried,' she said. 'Only thrice have you set the Ring upon your finger since you knew what you possessed.  Do not try.  It would destroy you.  Did not Gandalf tell you that the rings give power according to the measure of each possessor?  Before you could use that power you would need to become far stronger, and to train your will to the domination of others." Only someone truly powerful like Galadriel or Gandalf could possibly make use of that power to it's full, but even then, as the power comes from Sauron's spirit, that spirit would still have an effect, and corrupt the user.