Heat Margin shows you where you've been editing your code!
Visual studio has a feature built in to that shows you where you've edited your file (green for saved, yellow for unsaved). But over time, your entire document will show the green bar!
With heat Margin as you edit lines, indicators are added in the scroll bar and next to the line numbers (much like the default green and yellow makers you're used to).
The difference with Heat Margin is as you edit more lines, previously edited line indicators start to fade. You'll quickly start to build up an idea of where you are working!
Works with 2015, 2017 and 2019.