If you've played FFXIV, then it's pretty much a version of the Sightseeing Log from there, just as a plugin and website.
If you've not played FFXIV, then it's a plugin and website, that lets you hunt around for spots in the glorious game of Guild Wars 2 based on hints, and it'll tick them off once you've found them. The idea is to try to stand where you think the picture was taken, or where the hint suggests.
First off, you need to install the plugin, Dessa, which you can find instructions on how to do right here.
Then, just come back here, and choose a challenge, and click on it.
If you keep the page in the background (or another monitor ideally), you can then run around in Guild Wars 2 and hunt down those locations, and see over on the page as they get ticked off.
Think a challenge is good? You can like it (up to 10 times) to show your appreciation!