Hi!
Firstly, here is your SkyEnterprise one stop techie shop! https://www.juniper.net/documentation/product/en_US/juniper-sky-enterprise
Now onto the task at hand!
Basically, SkyEnterprise is a cut down version of on-premise JunOS Space and JunOS Security Director in the cloud.
AppTrack Analyzes application data and classifies it based on risk level, zones, source and destination addresses. Tracks application usage to identify high-risk applications and analyze traffic patterns, improving network management and
control. It will be empty because there isn't any high-risk apps or traffic patterns identified I would assume.
I think that answers everything, if there is anything else reply and let me know 
KR
Adam