From the site:
New constitutions are written every year. The people who write these important documents need to read and analyze texts from other places. Constitute offers access to the world’s constitutions that users can systematically compare them across a broad set of topics — using a modern, clean interface.
The site itself is pretty easy to use. You can browse by countries, which will allow you to download the full text of a countries constitution or you can enter a specific topic (i.e Freedom of Religion) and compare between all the countries whose constitutions mention the topic. For the record, I took a quick look at a few topics and a couple constitutions; maybe it is just my American jingoism but the US Constitution seems to be the best written and most elegant by far.
(h/t Hacker News)