Tempo = Board Advantage / Mana
Mana could either mean the total amount of mana you could have spent in a game up to each turn, to get an aggregate 'total' of Tempo for an entire game; or mana in one turn, to get 'marginal' Tempo change per turn.
Board Advantage means the power of the cards you have on the table to affect the game. We've realized for a while that Card Advantage does not improve Tempo. Putting important permanents in play is fundamental to Tempo. Removing or neutralizing your opponent's game-affecting permanents reduces their tempo without increasing yours, slowing the game down.
I'm intentionally leaving this more vague than it should be to avoid saying more about Tempo than I feel qualified to.
