An expression meaning ‘May Allah accept [your action]!’. 

Used after an action to request Allah’s acceptance of that action. For example, if a person states that they fasted for the day, one could respond with taqabalallāh. Taqaballāh is sometimes intended as a portion of a longer phrase ‘Taqabalallāhu minnā wa minkum’, which means ‘May Allah accept [the action in question] from you and from us’.