Why is it difficult to drag a heavy object along the ground, but so much easier if it is on wheels? Why is it easy to slide things along some surfaces but not others? The answer is a force called friction.
When two things rub together it causes friction. Friction slows things down and stops them moving. The amount of friction there will be is determined by how much contact there is between the surfaces of the moving objects and by how rough or smooth the surfaces are.
A wheel or a ball roll freely over a surface because only a small part of it is in contact at any one time. An icy surface is slippery because it is so smooth.
Try out this worksheet to test what you know about friction.