Gflags is a commandline flags library used within Google. The commandline flags are specified by users on the command line when they run an executable.
Typically, an application lists what flags the user is allowed to pass in, and what arguments they take -- in this example, -l takes no argument, and -f takes a string (in particular, a filename) as an argument. Users can use a library to help parse the commandline and store the flags in some data structure.
