From avira.com
What do the terms fragmentation and defragmentation mean?
One of the main possible culprits causing your computer to gradually crawl along is a fragmented hard drive. This means that pieces of files have been stored on the hard drive in different partitions — or, in tech terms, logically separated parts of a hard drive. This can happen over time if you change or delete files, creating gaps on the hard drive.
New files are then saved automatically in the resulting gaps. If the gaps are too large for the new files, they are split across several gaps — resulting in a fragmented file. This isn’t a bad thing in itself, but it can slow down your device. That’s because if you open a fragmented file, the computer has to read it from different locations on the hard drive to display its contents. The same applies to programs, which can also lead to 100% disk usage, slamming the brakes on your device’s performance.
You can piece together those fragmented files by defragmenting your computer. Your computer will then rearrange the files and programs on the hard drive so that all the fragments sit one after the other. This allows the computer to access the file or program more quickly. It also saves your hard drive from having to spin up and go hunting for file fragments, and can increase its lifespan. You can also use the CHCKDSK command to scan and repair your Windows hard drive.