On sport climbs, bolts are fixed into the rock (usually via a drill and glue). The bolts provide a safe point for the climber to clip a quick draw to, which in turn allows the rope to be clipped. The fixing of bolts to climbs is pretty controversial. Some feel that bolts add safety, while others see it as permanently damaging the rock.
Bolting is often used to open up routes that have no means of using regular protection and is frequently used on routes set in quarries. What ever your opinion of bolted climbs, it’s worth noting that it is often difficult to check the age and condition of the bolt until you’re already on the climb, therefore especially in the early stages of your climbing, always climb with someone experienced.