Tippet is the very end part of your fly-line set-up. It's the piece that hooks your fly to your leader. Tippet comes in various diameters (3x,4x,5x,6x...). The higher the number means the thinner the line will be. Tippet like 1x, 2x, is better suited for larger flies and rougher water conditions.
Let's say your fishing is a small pond and the water is glass and you can easily see through the water. You'll want something appropriate for the size of fly you are using, but you don't want something too thick that can be easily visible by the fish. So, you can see why there are different diameters. It all depends on water conditions, the fish you are going for, and the fly you have on.
Tippet material is either monofilament or fluorocarbon. I reccomend fluorocarbon! Although it doesn't float as well, its more durable and less visible under water.
