It's when an internet user *doesn't* realize that:
a) U'RE is a lousy abbreviation for YOUR, as they're both 4 characters long;
b) U'RE would more likely stand for YOU ARE; YOUR and YOU ARE are two different things.
When you're trying to type quickly and use "ur" in a sentence and somebody corrects you saying it's in fact "you're", you sarcastically reply with "mb, u're".
Like c'mon, when you're cutting down the word to type faster, the application is obviously supposed to be read within context and not needed to be corrected. In fact if you replaced "your" with "ur" instead nobody would ever correct you i.e. they'd only do it on the "you're" instance.