Keywords are used by portage (gentoo's package manager) to determine if an ebuild will be compatible for a given CPU architecture. In this video, I go over how to use the /etc/portage/package.accept_keywords file to mask and unmask packages on the basis of their keywords!
Interested in Linux in general and Gentoo specifically? Then check out my YouTube channel for more videos on these topics: https://www.youtube.com/channel/UC8F_ci7-G_XBXvoRGQR-fMQ