"The salesman explained to us about the new product."
Is the sentence grammatical?
Whilst I don't actually disagree with bhaisahab here, I generally find "to explain" in conjunction with "about" quite clunky.
I would prefer "The salesman explained the product to us", "The salesman explained how the product works to us" or "The salesman explained to us how the product works".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.