(Optional) Equipment, like living creatures, can be destroyed. Each piece of equipment will therefore have a number of hit points. When the item has taken enough damage, it will become unusable.
Most items will only take damage from being directly attacked, which does not generally happen ("Die, vile pair of shoes! Take that, you hideous tie!"). Of course, there are exceptions.
Some types of items may be repaired, while others cannot. Once a glass is cracked, it generally stays cracked (although it can be glued back together, and maybe even fixed with some technique still to be discovered). However, armour can often be patched. Your common sense should be able to determine what can be repaired, but the time and cost to do so is up to the GM. Many items require professional assistance to repair, while others can be fixed by almost anyone.