The specification is PSGI, the implementation is Plack, and it's well documented here: PSGI/Plack. Note that the tagline on that page is, "Superglue interface between perl web application frameworks and web servers, just like Perl is the duct tape of the internet."

While most one-sentence summaries fail to tell the whole story, you can take from this one that Plack is a middleware that sits between the webserver and the application, and that often the application is a framework that hooks its back-end into Plack, and provides a platform for a higher-level application. It's possible to write an application from scratch that interfaces directly with Plack, but it's usually easier to sit a framework on top of it. However, you can comfort your one-track mind in knowing that it is possible to program an application to directly sit on Plack.

I doubt there's any better source of information than that page and the links it provides. But you can also talk to people on the IRC channel, as referenced at the bottom of the link provided in this post.