Unsigned apps that Microsoft's cloud-based security service are not familiar with are considered untrusted. The first determines whether the app is digitally signed, the second takes usage into account. Microsoft uses two main factors to determine whether an app is untrusted or not. Untrusted applications are not necessarily malicious.
They may do all sorts of unwanted things on a PC, including deleting files, pushing remote control software on devices, stealing data, monitoring user activities and more.