Windows 64-bit Disk Defragmentation is a process that rearranges fragmented data on a hard drive, allowing the operating system to access files more efficiently. Fragmentation occurs when files are broken into pieces and stored in different locations on the disk, which can slow down computer performance.

Defragmentation consolidates fragmented data, placing related pieces of files closer together for quicker access. Windows 64-bit operating systems have a built-in Disk Defragmenter tool that can be scheduled to run automatically or manually. It analyzes the disk, identifies fragmented files, and then rearranges them to improve system performance.