What does it mean when a guy grabs/pats my knee?

[NathanDavis]

This is something normal, when it comes from family (especially older relatives) or very close friends (from the same gender).
I really don't know this person but. He might be older than you and a bit ouf of place, thinking that he could do this (which in a way is wrong, I wll not call it harrasment, but this is just wrong to do)
And there is also the chance that he thought you were very vulnerable at the moment, and then he took advantage of this, testing if he can touchy with you when you're vulnerable.
If this happens again, you have to say something, the best way would be "Can you please not do that?" and then see his reaction, if he gets upset or tries to justify and keeps repeating something over and over, it could be not sincere, an immediate realization that he crossed a line and realizing that he made you feel weird should be more natural ( but this is not 100% accurate)

[spartan55]

Uh, that is COMPLETELY inappropriate on his part. I'm pretty shocked you need that pointed out to you.

[btbc92]

He is being like a father figure, it is a man's way of saying that he has empathy with you. It is a gesture done to men and women. Since for a man, a woman is seen as the weaker sex, he taps the knee to show that while he will never overstep his boundaries, he wants to physically show he cares without again, overstepping. The same gesture is done to men, but they pat the back or shoulder. If he is older, it is a mentor/father-like relationship. As a friend, a guy does that to show that while you are not that close of a friend, you are still in his thoughts. Since your friends, this again saying you in his thoughts and you can depend on him for anything. He was being friendly and comforting. That's all. You can tell by sensing it.

[Holisticapproachsara]

He finds u attractive. But this is like sexual Harassment if u didn't want him to do that

[Avicenna]

He’s interested in being intimate with you