PLUGIN_CONTINUE and FMRES_IGNORED mean you wish to not interrupt any original action. For example, in a command function, this will make the game thing that the command was not found.
PLUGIN_HANDLED and PLUGIN_HANDLED_MAIN will block the parent action.
Much of the time you never actually need to return these values unless you know you have to. Timers, client connection/disconnect forwards, et cetera, do not require them. Command functions do.