That's a Letter of Assurance.
USMMA displays these on the portal. Unlike some of the other academies, USMMA also displays on your portal whether your CFA is a pass or fail immediately after evaluation rather than having someone reach out to you if you've failed. All of the academies do things slightly differently even though the application process is similar for each.
An LOA is basically a contingent acceptance, provided you meet medical, physical, and academic standards and obtain a nomination (USCGA is the only Service Academy that the nomination doesn't apply to), and also don't get arrested or let your grades significantly drop, or get injured...
You get the picture. Just get the rest of your package done, keep your grades and physical strength up, and don't do anything stupid!