Concurrent Assignments are one of the ways you can schedule the Assignments in your Trip. You might notice it when creating a Trip on Obzervr Manager.
What does it mean?
All your Assignments start at the same time. Notice that the Start time of both the Assignments is 25/01/2019 10:16AM.
All your Assignments start at different times, incremented by the Duration. Notice that the Start Time of the second Assignment is 25/01/2019 10:46AM and the first Assignment is 25/01/2019 10:16AM.
So that's what concurrent Assignments are! I hope that helped!