It's a language where you use your hands instead of your voice to make words. Like how people wave to say hello, except there are thousands of different signs to mean different things as well as quick ones to spell things out. There's actually a lot of different sign languages from different parts of the world, the ones I know best are American Sign Language, which is probably the most common, International Sign, and Plains Indian Sign Language, which has very few native speakers left and most of them are getting pretty old.