A tranche is a piece or a part of something, usually money. An installment of a loan is a tranche.
The noun tranche comes from the French word trancher, "to cut," which should help you remember that a tranche is a portion of something, not the whole thing. Usually, it's part of a larger sum of money, like a mortgage payment, half of a bonus payment, or an installment of lottery winnings. People who work in banking and finance use tranche to mean one bond or security within a larger financial deal.