These things are important. This page describes the process for installing security updates on Gentoo systems.
You'll need to run
emerge --sync to make sure your repositories are up-to-date. If this fails for some reason, try setting a different mirror in /etc/portage/make.conf and /etc/poratge/repos.conf/gentoo.conf. Respectively, you can try "http://mirror.rit.edu/gentoo" and "rsync://mirror.rit.edu/gentoo-portage".
Check for GLSAs
GLSAs are Gentoo Linux Security Advisories. Gentoo issues them for vulnerable packages. You can list ones that affect the system you're on by running
glsa-check -l affected. Note that this probably won't help very much if you haven't already synced the repositories (see above).
To read more about any GLSA, you can use
glsa-check -d <GLSA ID> where <GLSA ID> comes from the output of
glsa-check -l (it'll look something like "201612-25").
To fix all GLSAs that affect this system, you can run
glsa-check -f affected. If this fails, you can manually follow the instructions listed in