Whenever an object moves against another object, it feels frictional forces. These forces act in the opposite direction to the movement. Friction makes it more difficult for things to move.
Friction can be useful. For example:
Frictional forces are much smaller on smooth surfaces than on rough surfaces, which is why we slide on ice but not on concrete.
Friction can also be unhelpful. If you do not lubricate your bike regularly with oil, the friction in the chain and axles increases. Your bike will be noisy and difficult to pedal.
When there is a lot of friction between moving parts, energy is transferred to the surroundings, causing heating. Think of what happens when you rub your hands together quickly. The friction warms them up.
Bikes, cars and other moving objects experience air resistance as they move. Air resistance is caused by the frictional forces of the air against the vehicle. The faster the vehicle moves, the bigger the air resistance becomes. The top speed of a vehicle is reached when the force from the cyclist or engine is balanced by air resistance.
Racing cyclists crouch down low on their bikes to reduce the air resistance on them. This helps them to cycle faster. They also wear streamlined helmets. These have special, smooth shapes that allow the air to flow over the cyclist more easily.
Modern vehicles are also streamlined. Their smooth shapes make the air resistance smaller, which allows them to travel further on the same amount of fuel.