"McDonald's, I'm behind you." Tell me what this joke means.

This joke means a clever misunderstanding and pun.

First of all, this sentence literally means "I am waiting in line at McDonald's, and I am behind you", which means that the speaker is waiting in line to buy food, and the person in front of him/her (that is, "you") is also waiting in line.

However, the real meaning of this joke lies in another meaning, that is, "I" is behind "you", that is, "I" is slower or behind "you". This is a humorous expression, usually used for jokes or teasing between friends.

So the essence of this joke lies in this pun, which expresses two meanings with the sentence "I'm in line behind you": literal meaning and implied joke meaning.