Basically an object that would draw a geometric shape of choice - rectangle, triangle, circle, polygons, whatever, with a very small footprint and innate collision mask. Transparent or filled.
Why not just draw one in sprite? Well, if you need something like a circle with radius of 256 pixels, the texture takes 512x512 pixels regardless of content or blank space. Annoying. Plus it doesn't scale too well!
The uses are many - to simply draw circles, polygons or whatever for many purposes (range overlay, firing arc for turrets etc.) - should be scalable (vector!).
There you go. Up for the challenge?