The meaning of "I know, right?"

People use the expression, “I know, right?” as a response to what someone else just said, when they agree with it emphatically (strongly). Here is a good example of its usage, based on a conversation between two students talking about their math teacher, on Twitter:

Mia:  He is the worst math teacher. All he does is put up notes for us to copy without explaining them, and then give us homework.

Tina: And the notes never help.

Mia:  I feel like I have to teach myself everything.

Tina: I know, right? 

One more thing: When people use the expression, "I know, right?" in speech, it has a distinct intonation pattern, with falling intonation on the word know and rising intonation on the word right. 

I hope this helps.