In an approach an airplane is generally descending between 500 and 1000 feet per minute, that's a minimum of 8 feet per second. That may not sound like much but just hitting the ground at that rate of descent would lead to a big bounce and possibly damage the airplane. It would certainly be uncomfortable for the passengers.
In order for an airplane to land softly, it must decrease its rate of descent, to do this the nose is raised in a maneuver called a flare. A nose-up attitude will, in a tricycle configured airplane, bring the main (rear) wheels in contact with the ground first.
