You will get a new entry stamp in your passport.
The new stamp will show how long you've been admitted for this time.
Usually this will be 90 days, but the officer who admits you can decide to admit you only for the remainder of your original stay, if he considers that you're trying to live in the US under the VWP and just doing "visa runs" to reset your limit. That is quite unlikely to happen when you've been outside the US for a month, though.
(Note that the rules can also be read as saying that on each entry you must either be given 3 months or refused entry under the VWP. I am not aware of good evidence that they're being administered that way in practice, though).