Ytmb is an acronym which stands for the phrase: "You're the man, bro". It is often used in situations in which one person is trying to demean another's accomplishments/actions by using sarcasm. A common extension of the phrase is: "ytm fucking b", "ytmb, bro" or "ytm fucking b, bro". Although they don't necessarily make sense grammatically, they have become quite popular among young Americans today.
Friend 1: "Dude I drank like 24 tough-man sodas last night and totally got a hand job out in the parking lot behind a car."
Friend 2: "Ytmb"