The boundary activity report is the long-form log of all the boundary activity instigated by your trackers. This is different from the alerts report's listing of boundary entry/exit because here you see more of the actual data about the behavior of the trackers around the boundary.
We see which tracker, boundary name, time in (this refers to the time inside the boundary), entry date, entry time, entry location, exit date, exit time and exit location. Here you are getting information about what direction the tracker was moving when the arrived at a location, how long they stayed and what direction they headed when leaving. A great time to review this is making sure all your vehicles hit the job sites on time and making sure they stayed on site for as long as needed.
Narrowing down the data by looking at a specific tracker will give you a pattern of that specific trackers daily movement between known sites. If you filter by a specific boundary, you will see all the trackers that interacted with that particular location. Both options are very useful, maybe you need to see the behavior of a single delivery truck, to make sure it hits all routes. Alternatively, you might be monitoring the refueling station on your property, making sure the visits to the pump match the driving times.
The best practice here involves a high amount of set-up, making sure important sites have an accurate boundary. Once that is complete, this report provides all the details you need to make decisions about those locations.