Why does it mean when we say the best predictor of children's learning is what they already know?
The best predictor of a child's learning is what they already know.
The background knowledge or what is already known to the child is the foundation on which further knowledge can stand and hence acts as the best predictor of child's learning.
The child gains background knowledge or information from the parents and caregivers and comes to understand the world. The more background knowledge they have, the easier it is for the child to grasp new information, which is related to what is already known to child. For example, if a child knows what a bird is, he/she can make an easy connection to any bird. At school teachers try to make connections with the prior knowledge of the students and use this "familiarity" to build on the knowledge base. It has also been suggested many times that the new information should be presented in a manner that relates it to background knowledge.
Please note that prior knowledge should not be confused with in-depth understanding and it is the responsibility of the parents/educator to rectify any misconceptions. Prior knowledge's use as an effective classroom strategy is well-documented, and hence to ensure its success a child's background knowledge must be built by parents/educators.