In this short, the -s option is used to search section 4 of the system manual (i.e., "the man pages") using apropos instead of man for the intro command. In this example, the results include "networking", which can be found by running the following:

apropos -s4 networking

Or more simply:

apropos networking

Keep in mind that this option can also be used by separating the option using a space. The following would also be valid:

apropos -s 4 intro