The Fellowship of the Ring
The One Ring
The War of the Ring, the Lord of the Rings, the Fellowship of the Ring, the Ring Quest: this series begins, ends, and revolves around the Ring. So we can say with confidence that it’s the most important symbol of the series. What the Ring represents is power: power to control and influence things and other people. For those who genuinely don't care about power, the Ring doesn't really matter (Tom Bombadil, we're looking at you).
But for the 99.9% of us who are a bit greedy and who do want at least some kind of power, the Ring controls us. You know that saying, "all power corrupts"? Well, the Ring is power – just in tangible form – and boy, does it corrupt, too.
There are some protections against the Ring’s influence, but they're not very enduring. For example, love, which is the recognition of another person's happiness, can work well against a person's will to dominate. But the Ring (power) is so tempting that even love can't last forever. And as for the desire to do good, forget it. Even if you start with the best of intentions, the Ring will twist you until you're as nasty as Sauron... or close, at least. As Gandalf tells Boromir:
We cannot use the Ruling Ring. That we now know too well. It belongs to Sauron and was made by him alone, and is altogether evil. Its strength, Boromir, is too great for anyone to wield at will, save only those who have already a great power of their own. But for them it holds an even deadlier power. The very desire of it corrupts the heart. (2.2.225)
The Ring implies that power in any form, as good-intentioned as it may be, is bad news. This lesson is a useful one for Tolkien's characters; but it's also a nod to his readers. Remember, power struggles are everywhere: not just on Middle-earth, but on our earth, too.