Friction is the retrieve force acting between bodies that tends to oppose and damp out motion. Galileo was the first scientist we know of that really identified what friction was.
Friction is one of the parts of force that opposes motion.
Friction also that slows down moving things. In general static friction is greater than kinetic friction. Most of cases the value of coefficients of friction is between zero and one. Friction is not always bad. Friction is an important element that must be consider when designing a roller coaster.