The Ouroboros is a snake that eats its own tail.
You can also interpret it in the sense what the universe does, it does it to itself. You can't act on anything out of it and there is nothing acting on it. Enclosed circle. The snake is a symbol for what is earthly, animalistic, receptive, concrete and not abstract, something that crawls and lies in our unconscious.
The ouroboros is, like the circle, a symbol of being complete, full, eternal and at the same times this can only be because you are also finite, incomplete (like a snake seeking to bite something) and momentary.