I know what you mean
I understand and relate to or agree with what you're saying. They've been excluding me too lately, so I know what you mean about feeling left out. A: "Ugh, all this rain is so depressing." B: "I know what you mean—it's really been bumming me out."
I know (just) what you mean.
I know exactly what you are talking about, and I feel the same way about it. John: These final exams are just terrible. Bob: I know just what you mean. John: Why do we have to go through this? Mary: What a pain! I hate annual inventories. John: I know what you mean. It's really boring.