("Change mode") The Unix command and system call to change the access permissions of a named file. Each file (directory, device, etc.) has nine kinds of access which can be allowed or denied. Different permissions apply to the owner of the file, the members of the group the file belongs to, and all users. Each of these classes of user (owner, group and other) can have permission to read, write or execute the file. Chmod can also set various other mode bits for a file or directory such as the sticky bit and the set user id bit.
Unix man page: chmod