Self-Contained Systems are a software-architecture pattern, described on https://scs-architecture.org/. They are similar to micro services, but include a UI. They are full systems with everything, they do not depend on other things. They represent a single, vertical slice through the overall system.
One advantage, according to Eberhard Wolff as he describes on his video cast, is that a functional requirement can be solved completely within this system. You (usually) don’t have to cross developer teams when you change things; API may turn to be internal APIs if they are not required externally, and may be easier to manage.