According to Google it's a color filter that partially colorblind users can customize and use to improve color perception.
At least for me, I'm not sure it's doing much, but I am also finding the calibration routine it wants you to go through is fiddly and unreliable for me. Could my wide gamut displays be messing with it? I don't know, but I'm going to try it on another machine/screen tomorrow.
It may work for some though, and it's a nice idea. There are similar programs out there designed to enhance color perception of images, and a browser plugin is a logical extension of the concept. Accessibility options for programs to help colorblind users are not as common as they could be, and the web doesn't always design with the colorblind user in mind either.
Try it out for yourself, maybe it'll be helpful for you!