Basically, files on the hard drive can become fragmented. This is when parts of files are scattered around the disk. This can lower performance when accessing data, since the hard drive will need to seek areas which are spread about, when attempting to read a file.
Defragmentation (or defragging) resorts the data on the disk so that fragments of files are closer together (contiguous), thereby making data operations more efficient, improving performance.
|Desktop PC| |Macbook Air 13" (Mid 2013)|
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