Suppose you have a set of harmonics in 7 different tones. Each harmonica usually has a sticker with a letter indicating the tone. Sometimes this letter can be engraved on the body itself. Since you are a beginner, you will probably start playing with the most common C key. As you master the instrument, you can play other tones. If you use a self-study or video tutorial, you should get an explanation of which key to play a particular tune.