Friction is the resistance offered against the motion of one surface over the other. This is because, at the microscopic level, each surface has hills and valleys i.e. the surfaces are not completely smooth and hence the roughness of one surface locks over the other and restricts its motion. There are two kinds of friction: static friction and kinetic friction. When the object is at rest, static friction acts. While the object is in motion, kinetic friction acts. Static friction is greater than the kinetic friction because a larger force is required to change the state of rest to motion. You must have experienced that if you do not accelerate the bike while its in motion, it would slow down and eventually stop. In order to get the bike moving, initially static friction must be overcome and thereafter, kinetic friction to make it stay in motion.
