Disk defragmentation is a process that reorganizes the data on a computer's hard drive. Over time, files can become fragmented, meaning they are stored in non-contiguous spaces. This can slow down the computer's performance because the hard drive has to work harder to access these scattered pieces of data. During defragmentation, the system consolidates fragmented files and free space, making it easier for the hard drive to read and write data efficiently. This can lead to faster file access and improved overall system performance, especially for traditional hard disk drives (HDDs).