File fragmentation is a term that describes a group of files that are scattered throughout a hard drive platter instead of one continuous location. Fragmentation is caused when information is deleted from a hard drive and small gaps are left behind to be filled by new data. As new data is saved to the computer, it is placed in these gaps. If the gaps are too small, the remainder of what needs to be saved is stored in other available gaps.
Fragmentation causes slow access time because read/write head accessing the data must find all fragments of a file before it can be opened or executed. If the hard drive has to do this many files each time it's opened, it can decrease the computer's performance. The image below shows an example of file fragmentation. As you can see, the lower half has other files and gaps in-between what should be a continuous blue section.