SSO, or Single Sign-On, is a login mechanism where the user logs into a single account that is considered safe. After that it has the ability to use all the sites and services that are related to the login account without the need to log in once again. SSO is applied in a chain of services from Google and Bing.
Meanwhile, MFA is about asking for several identity confirmations before letting the user in. It is also about the login way, but supposes much less trust and fits better at the places that does not suppose the trusted login. Still, they may be combined (first you log in with MFA, then you can use all services with SSO).