SYNC

The SYNC variable - in /etc/make.conf - contains the rsync server you want to use when updating your Portage tree (the collection of ebuilds, scripts containing all the information Portage needs to download and install software).

A list of community mirrors is maintained on the Gentoo site.

Choosing a mirror
A good mirror can make Portage download source code faster. Usually, you can choose a mirror (with mirrorselect) that is near you.