A 5-on-3 occurs during basketball games when 5 offensive players come into their offensive zone while only 3 defenders are there to defend at that point creating a mismatch. Usually, this is a result of a fast break where the offensive team is able to move the ball up the court rather quickly without a good defensive recovery occurring. There are also 5 v. 3 drills used in practice that could be a situation known to be 5-on-3, however, the situation is usually a fast break mismatch in game action.
An offense with a 2 player advantage should look to pass the ball quickly around while looking to the inside where the defense will be left vulnerable after a couple of rotations. Open shots are the key to taking advantage of the undermanned defense as well as high percentage chances like layups and close shots.
The main philosophy on defense when left at a 2 man disadvantage is to defend the basket instead of individual men. Communicating amongst the defenders present in the game situation is also essential to make sure the offensive isn't given any easy opportunities at the rim or any good looks as best as they can until the rest of the defense is able to recover.