A tripcode is meant to prove your identity
You enter a 'password' after your name that only you know, like #ilikemudkipz, and it'll show up in your name as Anonymous !0Kjsdjhdh (or whatever random string of characters attached to said password.
Some people use tripcodes to have a certain word after their name. My usual tripdoe used to say !tHOMAS, because I am an unimaginative person.
In case you're curious, I don';t post with it anymore because my tripcode was made public, meaning anyone could impersonate me, and making it useless.