Today we learned about feudalism in history class.
You know that 主義 is "-ism." Add that onto an ism that is building a seal(ed) in system, and you have feudalism. Why? Because with feudalism, power is sealed in with one group or one family. Nobody else can get in (until they are overthrown, anyways), making this feudalism.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.