It has very little content of its own. Instead, it's a redirect service that provides relatively short, memorable URLs for index entries for a variety of Lisp software.
For example, you probably can't remember that the URL for section 126.96.36.199.2 in the hyperspec is http://www.lispworks.com/documentation/HyperSpec/Body/11_abab.htm. But now you don't have to! You can just link to l1sp.org/cl/188.8.131.52.2 instead. You can also link to symbols or format directives if you like.
This isn't a search service, it's for when you know the project and symbol you're looking for, and want to get to its documentation quickly. For search, try lispdoc.com.
L1sp.org is new, and will evolve. I don't recommend making l1sp.org URLs a permanent part of your software's documentation, for example. But I hope the service will be handy if you are chatting with a friend and want to easily point to some documentation.
If you have a suggestion for or question about l1sp.org, let me know.