What does 'you have my word' mean?

what does ‘you have my word’ mean?? and is there reply In English for that?

It means ‘I promise’.
There is no set reply. It depends on the context.

possibly replies might include:

“Thank you.”

“Ah, but is your word any good?”

or

“I’ll hold you to it” - (I’ll make sure you keep your promise)