Trivial constructivism
The simplest idea in constructivism, root of all the other shades of constructivism described below, is trivial constructivism (von Glasersfeld, 1990), or personal constructivism. In this principle (credited to Jean Piaget), Knowledge is actively constructed by the learner, not passively received from the environment.
This reacts against other epistemologies promoting simplistic models of communication as simple transmission of meanings from one person to another. The prior knowledge of the learner is essential to be able to "actively" construct new knowledge. Learning is work - effective learning requires concentration. There are some things you have to learn before others. The education system has always been built on a progression of ideas from simple to complex. Questions arise, however. What is "the environment"? What is "knowledge"? What is the relation of knowledge to "the environment"? What environments are better for learning? Trivial constructivism alone says nothing about these issues, and these are the shortcomings that the other faces of constructivism attempt to address.
References
Dougiamas, M. (1998). A journey into Constructivism, http://dougiamas.com/writing/constructivism.html
Von Glasersfeld, E. (1990) An exposition of constructivism: Why some like it radical. In R.B. Davis, C.A. Maher and N. Noddings (Eds), Constructivist views on the teaching and learning of mathematics (pp 19-29). Reston, Virginia: National Council of Teachers of Mathematics.