On older hard drives with magnetic memory (not the fancy M.2 and flash stuff we have now), you had to defragment drives.

Data was stored magnetically and you had to move a magnetic reader head over it to recover the data. Because the data took up physical space, it was faster for hard drives to store related data close together. As you deleted some files, the remaining files stayed in the same physical place. To make things faster, you would copy your files every now and then to condense them. Bringing all those fragments together was "defragging."

In modern drives, it takes the same amount of time to access all parts of your hard drive, so defragging is no longer necessary.