Ablock of wood is that rest on a wooden ramp. when wheels are attached to the block of wood, it moves down the ramp. explain each situation in terms of friction

So, without wheels, the statical friction is enough anule the gravitational force that is "pulling" the block down.

Now the wheels are special in this cases, when the friction on the ramp and the gravity pulls in different directions, but in different parts on the wheel (the friction is in the bottom, where it touches the ramp, and the gravity force is in the top, where is the block). This causes the wheel to make "rolling motion" which is a combination of translation and rotation. Because of this, the block with wheels will move down the ramp.

You should notice that, if the ramp is frictionless, there is no rotation in the wheels, just translation.