Baby sign language is a tool of communication that you can give to your pre-verbal baby. Babies begin to gesture at a fairly early age; putting their hands in the air to be picked up, pointing at things they want etc. They quickly learn that these movements will elicit a reaction from his/her parents.
Taking that natural gesturing to the next level, parents have started to teach their children specific signs that they can use to communicate their needs.
Most people use signs taken from a real sign language such as ASL in North America. By using ASL signs with babies you are not teaching them the actual language, you are simply using the exact sign for the exact English word.