The tap command allows Homebrew to tap into another repository of formulae. Once you’ve done this you’ve expanded your options of installable software.
These additional Git repos (inside /usr/local/Homebrew/Library/Taps) describe sets of package formulae that are available for installation.
E.g.
brew tap # list tapped repositories
brew tap <tapname> # add tap
brew untap <tapname> # remove a tap