There's an excellent description of the subject in
http://www.monmouth.com/~jsd/how/htm...html#sec-flare
Basically it's the stage in landing when you raise the nose to move from a nose down descent attitude to the nose high attitude adopted for contact with the runway. The nose gear of most aircraft is not strong enough to absorb the force of a landing. By flaring, the aircraft lands on the rear landing gear, which (should) be able to stand the force of the landing.