I'm new to this. How does this work and where do I go to check if someone is blocking or is invisible to me?
The usage of the application is simple. Install it, then input an ID and it will tell you if it's online, offline or invisible. There isn't a special procedure for this type of applications.
It is a spy app like iKeyMonitor, which can invisibly track activities on your Phone. But I didn't use it, so I don't know if it is safe and reliable or not.