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 remaining 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 for dozens or hundreds of different files as a program is opened, it can greatly decrease the overall performance of the computer. In the picture below is an example of file fragmentation, as can be seen the second example has other files and gaps in-between the continuous blue section.