This is when you buy a Put and sell another Put with a lower strike price. You would only use this spread if you are expecting the underlying stock to move up moderately because it will help you to benefit from the time decay.
For example, let's say you are anticipating a moderate rise in AAPL, when you put on a Bear Put spread it would look like this:
BUY TO OPEN AAPL JUL 15th 139.00 PUT &
SELL TO OPEN AAPL JUL 15th 137.00 PUT
You will receive a credit from the difference between the premiums of the two contracts.