Friction: the force that tries to stop materials sliding across each other. Sometimes friction is reduced by ball or roller bearings on wheels, oil or grease to make the surface slippery.
Static and dynamic friction
When the block is pulled gently, friction stops it from moving. As the force is increased, the friction rises until the block is about to slip. This is the starting or static friction. With the greater downward force on the block, the static friction is higher. Once the block starts to slide the friction drops: moving or dynamic friction is less than static friction.
Dynamic friction heats materials up. When something is moved against the force of friction, work is done and kinetic energy is changed into thermal energy.