The "2Do" application shows a blue dot in app's titlebar on macOS - what is it for?
The button is specific to the app &quot;2Do&quot; which you're running. It is not a general macOS &quot;thing&quot;.
You are not the first person to ask why the meaning of the dot is, or what it does (if anything) when pressed. For example there's a Reddit thread about it here:
https://www.reddit.com/r/MacOS/comments/aw08z5/what_does_this_blue_button_next_to_the_traffic/
However even though the manual for 2Do shows a screenshots with this blue dot - it is not explained what the purpose of it is.
I would suggest contacting the support for 2Do to find out more!