self-identification as a retard; so you quickly & easily know who your retarded friends are.
Break it down:
I know. -- declaration that you KNOW
right? -- questioning the above, in other words, you DON'T know!
Jim: People who say I know, right? Or text IKR? are fucking retards!
Jim: (deletes Tim)
Timmy: What does IKR mean?
Tommy: I don't know.
Timmy: Jimmy, what does IKR mean?
Jimmy: I know, right?
Timmy: Oh, Tommy, it means "I know, right?".
Tommy: I hate it when people say that!
Timmy: I know, right? It's so annoying!