Subjugation is like oppression or conquest: one group takes control over another and forces them to do as they're told.
Subjugation is one of many types of injustice in the world. It has to do with one group of people dominating another group by taking away their freedom. When slavery was legal in the U.S., that was a clear-cut case of subjugation: African-Americans were forced to live without rights, under the control of their white owners. To remember this word, think of its Latin root subjugat, which means "brought under a yoke."