When you fail to make a contract, you pay out a penalty for each undertrick, 50 each if you have not made game, 100 each if you have. If the other side doubles the contract, the penalites increase, sharply for contracts that go down by more than one trick. Consult a scoring table for the actual numbers. Suffice to say that going down several tricks in a doubled contract is a disaster unless the other side can make a game or slam and has the ability to bid and make it.
Redoubling doubles the increased penalties.

Successful Doubled Contracts

Doubling doubles a contract's trick score, and the increase counts towards game. Successfull doubled contracts of 2 Hearts or higher qualify for game. Successful doubled contracts earn the small "insult" bonus of 50 points.
Doubled overtricks count 100 apiece if the offense has not made game, 200 otherwise. Redouble doubles trick score, the insult, and any overtricks.