It sets the shell variable
RUNNINGAS equal to the output of the command
/usr/bin/id -un. On most systems, this would be your username. Note that since, as far as we can tell here,
RUNNINGAS has not been
exported, the value of the variable would not be available to subshells started from this shell.
You can use
man to get information on the
id command (
man id). Most other commands also have manpages, which are a good resource for learning what the commands are for and how to use them and their various options.