Rolling Resistance
Rolling resistance is the force that slows a wheel down as it rolls across a surface.
Think of it this way
Roll a ball across a wet cloth. The cloth grabs the ball and slows it fast. Roll the same ball on dry glass. It keeps going much longer with less drag.
Explaining rolling resistance by grade level
Push a toy car on carpet, then on a smooth floor. The car stops faster on carpet. The soft carpet grabs the tire more, so the wheel slows down. Hard, smooth floors let the wheel roll much more freely.