Constructivism is the idea that we "construct" our own knowledge. It is both a simple and complex theory. Simply put, our ability to learn is dependent on our creating our knowledge, best done in a hands-on or applied way, basically saying that we generate our own learning. It is a complex theory because it is dependent on a knowledge of how the brain really works, which involves how we take in the world, through our senses, and how that sensory input gets moved along to our minds and in our minds, through the connections between our neurons, creating new, literal pathways of learning.
Let me contrast for you two learning situations. In one situation, a professor stands in front of the class and gives a lecture on writing a persuasive essay. The students take notes and then go off to write their essays. In another classroom, the professor hands out a sheet of guidelines, puts the students into groups of two or three, provides a choice of essay topics, and directs the students to write for 15 minutes and then critique one another. Which class do you suppose is better prepared to go home and complete the assignment?
The second class has not been sitting passively listening to someone. They have done some actual writing, tested it against an audience to see how persuasive it is, and maybe even done some revision. This is the class that is better prepared. All the students had to construct their own knowledge of essay-writing by grappling with the writing and by relying on feedback from their environment to test their efforts.