The diagram above shows the an animation of a rotating cam and flat follower.   As the cam rotates the the follower is pushed up and down.

There is some external force pushing the cam back down, so that it remains in contact with the cam profile.

The cam shown above is also known as a plate cam.

This type of follower can be found in the cam and follower system used to open and close inlet and exhaust valves in an engine.