To Pile Up, To Liven Up, To Stir Up
Transitive verb and Ichidan verb
もりあげる
俺の友達はメチャクチャ面白くて、パーティーを盛り上げるのが得意なんだよ。
My buddy is ridiculously interesting and he’s great at livening up parties.
盛る is "to heap up." If you "raise" (上げる) what you heap up, well, that comes with heaping something up, making this word to heap up or to pile up. This word is also used when you want to liven up some things. Heap up that energy and stir things up!
The reading comes from 盛る and 上げる. Know those words and you'll be able to read this one too.