It's a condensed version of your code using color blocks instead of text.
The Sublime text editor used a similar view along the right side of the editor. You can use it to quickly navigate to a section of code without having to read each line.
I was inspired to make this by the minimap package for Atom, which is a copy of Sublime's behavior.
In case anyone cares, this is an option in Visual Studio too. Under "Text Editor > All Languages > Scroll Bars", you have the option to "Use map mode for vertical scroll bar". Can also be configured on a per-language basis.