there are some common flags, like -v means -verbose or -version, -l means long format, -h help, but you always need to check the manual page, because they may have different meaning.
The - (minus) sign usually means the actual process will use stdin as input instead of a file. Is accepted only where it has any meaning (df need no input file).
see for example man grep:
grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus (-) is given as file name) for lines containing a match to the given PATTERN. ...