Hi Matt, welcome to the forum!
What you are seeing there is the range between the civil and astronomical sunsets/sunrises. The difference between civil and astronomical twilights is basically how far the Sun is below the horizon. To get an absolute dark sky you need to wait for the astronomical night, but for other purposes the civil night will suffice - that’s why they are both represented in that small chart.
The following image shows the exact difference between civil / nautical / astronomical twilights:
For more information about civil/nautical/astronomical twilights, you can visit this page: The Different Types of Twilight, Dawn and Dusk
I hope this helps!