This idea of a skill "building on another" can be compared to building a home. First, you need to put a foundation for the home or building. Then, you can begin to stack bricks onto the foundation. Some skills "build on another" because you need to have the foundation first. Then, you can begin putting skills on top of this foundation. Another way of describing this is like teaching a child to eat. You should not put a big piece of meat on the plate; instead, you begin with small, soft spoons of liquid. Then, you can build on this skill by offering larger amounts of more solid food as the child learns to chew food properly. In the context of your specific sentence here, it means that you will learn phonics best when you practice each day. On day one, you will learn things that will help you on day two. Your lessons grow or build up as you continue learning and practicing.