system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 What is defragging my hard drive, and how will it help my computer?

This is probably a stupid question, but I know nothing about computers.
gpt
Here's the thing with hard drives - they're optimized to write / read as much stuff, as fast as possible.  Imagine a guy trying to stack boxes, and they just keep throwin' more boxes at him, all the while someone else is asking him about this box or that box, where'd it go, all that.  He just needs to keep up with the in/out flow: *how* he's storing things is secondary.

Now, when the guy gets is a shipment of 4 boxes that should be grouped together, he's gonna do his best to put them in the same place.  But in the course of all the in and out, maybe those things get split up.  Or maybe they add another 4 boxes to the first 4, and there's no room to put them all together, so they get all spread out.

That's fragmentation.  If a file (group o' boxes) gets all spread out, then it might be tougher for the hard drive to read it all off at the same time, together.  So defragging is (to go back to our analogy) that guy coming back into the warehouse one night when no one's bugging him to take the time to organize everything the way it should be: put all the stuff together that should be together, get all his space in order, all that.