Rage Quit, as it's usually called, is when the majority of the losing team quits the server. This may be due to excessing Base Rape, unfair odds, or simply frustration from having lost. Regardless of the reason, those who "rage quit" are angered at the server and/or its players and hope to find a friendlier server, hence the term "rage".
It is generally frowned upon, and results in a massive drop in the number of players. This leaves the remaining members of the losing team at a disadvantage, and gives the other team less of a challenge. This sometimes results in the Pregame, where no XP can be earned and stats are not recorded.
It is usually not considered "rage quit" if only one player quits. Rage quit usually has to involve two or more players from the same team.
On the other hand rage quitters create lob-sided server, because the "Matchmaking" system puts new players on the empty side. Often they are scared of the high ticket advantage of the opposite team and quit immediately again. That's the reason why many players aren't pleased by the Matchmaking: The chance to be put in an unbalanced server is much higher than getting in a balanced game or even impossible to get in a filled one of course.
To avoid this situation you should join your bookmarked servers or friends.