Understand homebrew and keg-only dependencies

For a software to be "keg-only" means it is installed in /usr/local/Cellar but not linked into places like /usr/local/bin, /usr/local/lib, etc. That means other software that depends on it has to be compiled with specific instructions to use the files in /usr/local/Cellar. That's done automatically by brew install when a formula specifies keg-only dependencies.

Formulas that specify keg-only dependencies make sure that the equivalent system libraries are not used. Your installation of vips is linked against a specific version of pixman in /usr/local/Cellar/pixman/<version>, so it isn't affected by the system version of pixman or any other Homebrew versions of pixman you might install.