As Webster says:
A strong pin, of iron or other material, used to fasten or hold something in place, often having a head at one end and screw thread cut upon the other end.

In climbing, bolts have had a contentious history. A bolt on a climb makes the climb very safe. As you climb, you get to the bolt and clip your carabiner straight into it. There is no messing around with gear and you can place a bolt on an otherwise blank face of rock. However, to place the bolt, one usually has to drill a hole in the rock and then glue the bolt into the rock.

'For bolts' is the safety argument. 'Against bolts' is the idea that you should take only photos, leave only footprints. Bolting a climb changes the character of the rock. Before the introduction of bolts, if someone wanted to do a dangerous climb, they just did it. If they didn't have the balls to do the climb, then the climb would have to wait for someone who did.

The issue of bolting has become a holy war in climbing. If you bolt without consensus, you are likely to find that some groups will have come along and removed or chopped off the bolts.

Personally, I am of the ethic "the only good bolt is a chopped bolt" and "when in doubt run it out."