Primarily, it's a small license file that unlocks features within the free Nova Launcher app. The two are designed to work together, and updates to the main launcher happen through the free app.
In APK-distribution slang, “Top” often indicates: nova launcher prime 62kb apk top
The standard Nova Launcher app contains the actual code, user interface, and features. It is usually around 10MB to 15MB. Primarily, it's a small license file that unlocks