USE flags define what functionality is enabled when packages are built. It is not recommended to add a lot of them during installation; you should leverage Funtoo Profiles instead as much as possible and only set as needed. A USE flag prefixed with a minus (" - ") sign tells Portage not to use the flag when compiling. Through use flags we generate more secure stripped down binaries with reduced attack surface and better performance. A Funtoo guide to USE flags will be available in the future. For now, you can find out more information about USE flags in the Gentoo Handbook.