First, congrats! Being offered a contract is a big deal!
But I agree with what Debbie said. Be certain this is a publisher you want to work with. Do they publish good quality books? Are they asking you for any money? (If so, run. That is not a traditional publisher.)
If this is a publisher you do want to work with, send the other publishers an email with "OFFER on NAME OF BOOK" in the subject line. Then give them a deadline to respond to you, a week or two is normal. If they don't respond, consider it a NO and go ahead and sign the contract you have! You do not want to wait too long.
(If you are 100% certain that this is the publisher you want to work with, even if the other publishers offer, too, you could just sign the contract now and send an email withdrawing your submission.)