Flanking

From TF2 Wiki

Flanking is a maneuver that by which players attack from the sides or the back. Attacking from the front is a more direct, yet more hazardous method of attack due to the fact that this is where the enemies are focussed. Most enemies are not protecting their flanks, so although it may be a more indirect route, it will usually catch the enemy off guard. If they do not initially notice that you are there, you will have an opportunity to get a hit or two in before they can react, killing them or softening them up for the frontline attackers.

Example:

A troublesome Heavy/Medic team are attacking your front lines. Your defense is taking a hammering. While supported by a friendly medic or with a carefully maintained sentry gun in place, the attackers can be held off and their attention detracted long enough for the 'ambush' classes to take out the enemy Medic, allowing the Heavy to be finished off either by the ambushers or the frontline.