You will receive a special, beautiful certificate with your name and the Parshah (Torah section) in which your letter appears.

Why won't the certificate specify the actual letter?

Every letter in the Torah is equally special and holy. But some children may be disappointed that they got a less "prestigious" letter than their friend did (one child may get a letter in Moses' name, for example, and another in Pharaoh's name). That's why the certificate only specifies the Torah section in which your letter appears—so that no child should feel more or less important than another.