Gravity is a conservative force. Objects launched upward near the surface of the Earth slow down for awhile, losing kinetic energy but then begin to fall toward the Earth, speeding up and gaining kinetic energy. The force exerted by a spring is also conservative. In the second category, which we'll label non-conservative , we place forces which can only do negative work. Non-conservative forces can only reduce kinetic energies of objects.
Friction is a non-conservative force.
Objects acted upon by friction only slow down. They never speed up. If a conservative force takes kinetic energy away from an object, the object can get that kinetic energy back at some later time.