USE flags define what functionality is enabled when packages are built. It is not recommended to add a lot of USE flags during installation; you should wait until you have a working, bootable system before changing your USE flags. A USE flag prefixed with a minus ("-") sign tells Portage not to use the flag when compiling. 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.