Hard disks hold data in clusters.
A cluster can be no larger than 32K, which means many files need more than one cluster for storage. Larger files, such as data files from office and multimedia applications and the files that constitute programs themselves, get stored on multiple clusters of the disk. Ideally, these individual and related files are stored on contiguous clusters, because then the PC has to spend the least possible time locating their components. Over time, however, as you accumulate more and more files and as current files are increased or deceased in size, individual and related files become spread across noncontiguous clusters. Portions of an individual file or a collection of related files can be scattered all over the disk, and to load them the computer must locate and read each portion in turn.