What is Hegel's concept of thesis, antithesis and synthesis, in simple terms?
A couple of things to clear up before moving forward:
Hegel never used the terms thesis, antithesis, and synthesis. They were coined by Johann Fichte.
Hegel did, however, use this type of structure to advance his philosophy.
There is nothing simple about Hegel and the problem is exacerbated because he wrote in German. And so if you aren't fluent in both German and his use of terminology and his style of writing, you're going to miss the mark on what he's trying to say.
Ok - all that said, let's just talk about the underpinnings of what is understood by thesis, antithesis, and synthesis.
Thesis refers to an idea, usually an intellectual proposition.
Antithesis refers to the refutation of the idea.
Synthesis is the moulding of the idea and its refutations into a new idea.
For instance, I can crudely write an example like this:
Thesis - There is a God.
Antithesis - There is a lot of bad in the world.
Synthesis - There is a God but His ways are mysterious.