Hi Surendra.

Well, deep understanding is knowing why things are the way they are.

For example, let's suppose you are learning calculus and more specifically the derivative. You can learn to derivate by learning the derivation formulas. And the formulas work, but they are only pieces of factual information, and that's not deep understanding, since you are memorizing them. A deep understanding will be learning why the formulas are the way they are, where do they come from. And that questions may generate another questions and that's how deep understanding is built.

This is what Leonardo Da Vinci made. He wanted to know how things work and why they work that way.

I hope my answer has been helpful.