"ma mua" = closely translated to "before" in english, notice it uses "ma" particle (in/on/at) + " mua" (before, first)
"ka ʻai" = eating. This is understood to be the infinitive form of the verb ʻai = "to eat". When in this form, it is a noun-like usage in Hawaiian meaning " the habitual activity that is to eat." When you say " ma mua o ka ʻai" you are really saying something like "at the space/time called "before" of the habitual activity that is to eat." Which in English is "before eating" lol.
so now we have finally: "E pule ma mua o ka ʻai." Pray (in suggestion form) in the before-spacetime of the habitual activity that is to eat," or just "Pray before eating."
Hope this helps you! You really asked a loaded question lol.