The adjective effusive means “overflowing”, “extravagantly demonstrative”, or “unrestrained or excessive in emotional expression”. The verb form to effuse means “to pour out” or “to radiate”.
Wikipedia, of course, has a page about effusion. In chemistry, it’s “the process where individual molecules flow through a hole without collisions between molecules” (think of a deflating balloon).
The most common use of effusive would probably be to describe “effusive praise”, either from expert critics or from effusive fanboys. The word also comes up a lot in articles about the Academy Awards, what with all those gushy actors sobbing over their Oscars.