When a body translates, as well as rotates over a surface without slipping, its motion is known as rolling. The frictional force developed opposite to the direction of motion is called rolling friction.
Souce of Rolling Friction:
When a wheel rolls on a surface, its motion gets obstructed due to deformation of the wheel, or the surface or both at the plane of contact. In real life, usually a tyre and ground are not perfectly rigid and both get deformed to some extent.
Such deformation is easily noticeable when a heavy wheel rolls over soft ground. At the point of contact between the wheel and the ground, an indentation is created on the surface due to the weight of the wheel. The ground just in front of the wheel swells up slightly.
At the point of contact, the wheel also gets deformed and a slight flattening is easily observed. These deformations at the point of contact are the source of rolling friction. When the air pressure is low in a cycle tube, the tyre is flattered and the frictional force increases. If both the surfaces in contact are rigid enough, then the deformation is negligible, and so the rolling friction is quite low. For this reason, the friction is considerably low in the case of a train running over a railway track.
Note: The deformations of the surfaces in contact, in case of a rolling motion are temporary. As soon as the point of contact shifts, the wheel and the surface regain their original shapes and the deformation shifts to a new point of contact.