The "Notice of Allowance" is an official notification sent by the USPTO to those trademark applications that have formally passed the examination process. It also indicates that a given application passed the opposition period of 30 days with no extension request being filed.
After the NoA is issued, the applicant has six months to file a "Statement of Use". This statement must include a specimen - proof that your trademark is used in commerce in the US market. It can be a photograph of your product, packaging, a screenshot of your website, marketing materials, etc.
If the trademark in question is not used in commerce when the "Notice of Allowance" is filed, the applicant can file a "Request for an Extension of Time", which will grant additional six months for submitting the "Statement of use". Failing to provide a "Statement of Use" or requesting the "Extension of Time" might result in your application being abandoned.