Lift in KSP is a mechanical aerodynamic force generated by an object moving through an atmosphere. Like thrust, lift has both a magnitude and a direction. The location on a craft where the sum total of lift generated by all its parts balances out is the center of lift or CoL. Flight happens when the CoL is in the general direction of “up” and of a magnitude at least equal to the weight of the craft within the gravity of a celestial body.
Lift is contrasted with drag, the force directly opposing an object's motion through the air. As of version 1.0, all parts generate some lift as well as drag; however the lift-to-drag ratio is much higher from aerodynamic parts such as wings, control surfaces, etc. KSP also models drag created as a consequence of lifting force.
All else equal, lift increases with greater speed and decreases with higher altitude. The faster an object flies, the greater the mass of air it would encounter within a timespan and thus the greater the mass pushing against its own. Atmospheric density, or air pressure, decreases further from the surface of a celestial body, meaning less air to encounter and thus less mass contributing to lift.
The ratio of lift-to-weight must be equal or greater than “1” in order to successfully counteract the downward pull of gravity. Increasing speed can keep a craft's lift-to-weight ratio from becoming less than favorable while climbing into thinner atmosphere; but once its speed reaches escape velocity, this negates the need for lifting force to overcome gravity. This, in effect, is the goal of spaceplanes.