What are fuses used for?


I have, I’m sure to many, a very dumb question. I keep reading about fuses.  I have a fairly sophisticated system and have never come across the need for a fuse.
Can someone tell me where these fuses go and what they’re used for?
128x128rvpiano
If you have the manuals for your equipment it shows you where to replace a fuse. If you don't have the manuals you can probably download them from the manufacturer. 
Look on the back panel. Near the AC cord/outlet you will see a round (usually) screw-type cap. Below it will be a designation indicating the size of the fuse required. Unscrew the cap and pull it out. The fuse is held at one end by the cap. It is a press fit. Pull on the fuse to remove it. Hold the fuse up to a light and observe the very thin wire within. If it is intact the fuse is good. If the wire is broken the fuse needs replacement. Replace only with a fuse of the same rating on the back panel.
Some fuse holders are square and require a push-in for release. Others are round and require a push-in with a twist before they can be pulled out.