A beginner is someone who's brand new at something. In swimming lessons, a beginner practices putting her face in the water and blowing bubbles.
If you're a beginner, you're a novice or an amateur — in other words, you're still learning. When you're learning to drive, you're a beginner and probably need the help of an instructor while you drive carefully around your neighborhood. A beginner at speaking French might be able to say a few phrases and know some basic vocabulary words. The earliest meaning of beginner, from the 14th century, was "founder," or "person who establishes an institution."