Defragmentation is a process used to reorganize the data on a computer's hard drive. Over time, files can become scattered across the disk, making it harder for the system to access them quickly. Defragmentation rearranges these files so that they are stored in contiguous sections, improving the overall speed and efficiency of the computer. This process is particularly important for traditional hard disk drives (HDDs), which rely on physical movement to read and write data. However, solid-state drives (SSDs) do not require defragmentation, as they access data differently and are not affected by fragmentation in the same way.