If practice is to do something repeatedly to learn a skill, and theory is an abstract concept that has been thought about, praxis is the process by which the theory is enacted or realised. It can also refer to the act of engaging, applying, exercising, realising, or practising ideas. It is a kind of critical thinking that combines reflection and action in a cyclical, possibly iterative pattern: take action, consider the impact, analyse the result, revise, redo. 

The praxis (theory) Wikipedia page is very comprehensive.