When you rescue something, you save it from something bad — you come to the rescue. If Muffin, your kitten, is stuck in a tree, you can get out your ladder and climb up to rescue her.
A rescue is a good thing, much appreciated by people (and kittens). If your house is in foreclosure, you'd be very grateful for the uncle who comes to your rescue by paying your mortgage. Whenever there's an earthquake or other disaster, rescue operations are quickly put in place to find survivors. You can use the word more casually, as in describing how you saunter over at a cocktail party and rescue your friend from a conversation with an insufferable bore.