IKR stands for "I know, right?", a rhetorical question that communicates understanding and agreement. It is a way of saying, "I know exactly what you mean."
IKR is commonly used in response to a friend saying something that you really agree with. Perhaps an opinion that you have had for a while and are looking for someone to agree with you. However, it can come across as too forward if used in the wrong context
"I can't believe she was disqualified when she clearly didn't do anything wrong."
"Yeah, IKR?"