Work done by a frictional force: 

• The frictional force is the force that opposes the motion of a body.
• Kinetic energy is the energy possessed by a body due to its motion.
• When a frictional force is applied to a body, it opposes the motion of the body. 
• Kinetic energy possessed by the body is used to overcome the opposition of the frictional force. Hence, the kinetic energy of the body decreases and the body slows down.

Therefore, We can say that work done by a frictional force decreases the kinetic energy of the body.