Here is the breakdown:

• chmod is the command to change the mode of the files (-rwxrwxrwx)
• -R is the recurse flag,
• a means “all”, you can also have u, g or o (respectively user, group and others) instead.
• + is add (you can remove with -),
• rw is the permissions (rw of rwx)

I prefer changing permissions this way instead of using absolute values (0777 for instance) as it’s easier to reverse if you made a mistake.