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

Use untap

brew untap sethdeckard/proj