In a piece of music, poco a poco together with another instruction tells the performers to do something gradually, or little by little, such as speed up or become quieter. The gradual change gives listeners plenty of time to notice it.
Poco a poco is literally "little by little" or "bit by bit" in Italian — in other words, gradually. For example, poco a poco rallentando means "gradually becoming slower," and crescendo poco a poco means "gradually getting louder." As you can see, poco a poco can go either before or after the main instruction. Also, if you know any Spanish, you'll notice that the Italian for "little by little" is the same as Spanish!