Life to which we got used without friction force is impossible. We move, we stand still, the objects created by us do not break apart and do not slide on a surface, - all this thanks to friction force.
Friction force arises at contact of two bodies relatively each other. It has the electromagnetic nature and is caused interaction atoms and molecules with each other. Friction force is directed on a tangent to the surface of contact of bodies. It is also called force of dry friction and divided into the static friction, slidings and swings.
In the absence of the movement of a subject it is affected by the rest friction force which is equal on the module to force of external pressure and is sent to the opposite direction. If the external effort exceeds the maximum size of friction force of rest for this body, then the subject begins to slide, and there is sliding friction force which usually less than friction force of rest. For this reason the sledge it is heavier to get moving forward, than to carry.
Friction force arises also when swing objects. Friction force of swing is small. Therefore often neglect its action at simple physical calculations. If the body moves in the liquid or gaseous environment, then affect it force of viscous friction. This interaction is much weaker, than at dry friction. Notice that force of viscous static friction does not exist. Owing to that it is so easy to move the subject shipped in water. Friction force not always obstructs the traffic. For example, when walking there is rest friction force directed forward. It interferes with legs to slide back as it occurs if you try to move on slippery ice. Friction force gives acceleration forward. In this way wheels of the bicycle, the car, the train move. Action of brakes is based on presence of friction force. At the same time sometimes it is required to reduce friction. Details of mechanisms quickly wear out thanks to these forces. In that case lubricant which replaces rest friction force with a sliding friction, much smaller on the module will help.