A few posts ago I discussed trademark notices of allowance. On the patent side, a notice of allowance is a different instrument. It indicates that a patent application has been approved. When a patent application is filed, it is initially processed for formalities and then eventually examined by a patent examiner. The applicant works with the examiner to move prosecution of the application toward allowance. If an examiner issues a notice of allowance, that means that prosecution has, for all intents and purposes, ended and that the claims defining the scope of protection are allowable. The applicant and examiner, essentially, agree on the proper scope of protection.
However, a notice of allowance does not naturally result in the issuance of a patent. Rather, it indicates which claims can be allowed and the reasons they are allowable, or how they are different from the prior art. The notice of allowance provides a 3 month window in which the applicant can file final issue paperwork and pay the issue fees. If the paperwork and fees are lodged within that 3 month window, the patent should generally issue; if not, it will not.