Hello! Those expressions are "idioms" from Portuguese (Brazil), the Meaning depends the context and the region of the Country. I never heard "tais rezando", but I'm from São Paulo region surely this expression it's from other place. Could you be a little more especific? What kind situation did you heard those words?

Could you give me some examples or phrases which use these terms?

Anna, The sentecion "Que onda!" means "It's funny!" And "Tais rezando"! means "You are praying!"