See
http://www.nwa.com/plan/guide03.html. Normally you can also dig into the fares rules on your ticket and see language describing the 24 hour cancellation policy.
If you purchase your ticket at the time you make your reservation, you can secure your seat, the fare you've been quoted and a guarantee for a full refund without penalty fees if you cancel your reservation by the end of the next day (midnight of the departure city). If you don't cancel the reservation, however, you will not receive a refund unless you have purchased a fully refundable fare.
Note: Some special offers do not allow a 24 hour hold. These exceptions are noted in the terms and conditions of the special offer. Please read all conditions carefully before purchasing your ticket.