These markers allow you to arbitrarily mark pieces of text with pre-defined colors, which is extremely useful for task lists, checklists or the likes: you can easily mark an item like passed/failed etc.
Yes, the name is Highlighter:
Description from the home page:
With this extension you can highlight elements on the webpage. Basicly it works like a highlighter pen, but it's more complex. You can highlight any number of elements including sections of text and tables and areas on images.
- press hotkey and mouse button and draw the rectangle
- click on a rectangle to destroy.