What it means is that, as John Holt put it, learning is a product of the activity of the learner. That is, direct transmission of information from a teacher to a learner just does not happen. Children are not buckets into which we can pour knowledge. The school system is based upon what Popper calls the bucket theory of the mind. In fact, I contend that there is no such thing as instruction from without the structure, or the passive reception of a flow of information which impresses itself on our sense organs.
All observations are theory-impregnated.
There is no pure, disinterested, theory-free observation. We do not discover new facts or new effects by copying them, or by inferring them inductively from observation, or by any other method of instruction by the environment. We use, rather, the method of trial and the elimination of error. The fact is, when you give a few child a lesson you have planned, the result is entirely unpredictable. They do not learn by induction.