This comamnd is used to change the mod bits/flags of files and directories. There are mainly two ways to use this command: numeric or symbolic. The numeric mode is useful for setting the complete set of mod bits of a file or directories. However its use require a little more expertise and memorization.
The symbolic mode is useful for modifying just one or two flags, and it is easier for a beginning user.
For the most authoritative documentation on this command, use the man command.