What's the circle at the top of each list?

There's a circle at the top right of each list, to the left of the three dots. I don't even know what to search for in the help topics, because I have no idea what this is or what it does. It seems like some kind of filter because it changes my list when I tap it but I don't see how

Hi @Rebecca_C1 , thanks for using GoodTask.

It's a toggle that let you show/hide completed tasks. Also when you're in 'Focus' mode (that's managed by iOS), it should filter lists accordingly too and you can manage it through this button.

Thanks!