Cumulative upkeep is a cost that grows every turn. At the beginning of your upkeep, you put an age counter on the permanent that has it, then you must pay its upkeep cost once for each age counter on it. If you can't or don't pay the whole amount, you sacrifice that permanent. A card with cumulative upkeep {2} asks for {2} on the first turn, {4} on the next, {6} after that, and so on — the reminder text on the card gives the exact cost.
That rising bill makes these cards cheap to keep around early and expensive later. You get a powerful permanent for a small payment, then either keep feeding it mana each turn or let it go. Knowing when to stop paying is the real decision: often the card has already done its work, and letting it be sacrificed frees your mana for something else.
