If it slows to much, the plane stalls and quickly loses altitude. Pilots must keep enough speed to make sure the plane stays in flight. When a plane is on the ground, standing still in calm air, only the force of gravity is working on it. The plane can be pulled through the air by propellers or pushed by a jet engine.
Air is a gas, and air has pressure.
Picture the air surrounding a plane as putting pressure on various parts of the plane. Lift is a force of this air pressure acting on the wings of the plane. It is generated when the air pressure on the upper surface of a wing is less than the air pressure on its lower surface. The difference in pressures is lift.