This command is part of the posix extensions and needs to be imported to be available to the shell.
See Posix Commands
|-R,-recurse||apply mode changes recursively|
The first part of the mode string can be any combination of "u" for user, "o" for others or "a" for all.
The next part is either a +, -, or = meaning "add the permssions" , "remove the permissions" or "set the permissions".
The final part can be any combination of "r" for read, "w" for write and "x" for execute.
Multiple operations can be separated by a "," e.g.
chmod a+w,u+r file