The magnitude of frictional force is equal to the magnitude of the normal force multiplied by a frictional coefficient. The normal force is proportional to the force of gravity and is the ground pushing back up against an object. The frictional coefficient depends on the surface properties of the objects.
Frictional forces oppose movement and are caused by the tendency of the atoms and molecules in different objects to interact when they are very close. Rough materials cause more friction because the roughness results in more surface area, so there are more chances for the molecules to grab those of other objects. Air hockey tables have extremely low friction because the puck floats above the table, reducing the normal force to nearly nothing.