Could anyone help explain Model Driven Development in few simple words?

I have been researching Model-Driven Development(Architect), however, after reading a lot of papers, I still cannot have a clear understanding of it. It seems so abstract. Ex:

• What is its main idea?
• What is exact the model (it means the diagram, right?).
• MDD means we will design diagrams, then using the tool to convert to code???
• How can I apply it to my system, what is the step to apply... ...

Anyone please help give me some explanations?

Thanks a lot.