Hint; it's in the name (google).
googling for "google closure" first hit https://developers.google.com/closure/
Google closure consist of the closure library and closure compiler (and some other tools like soy templates and css language). Closure compiler is used to minify, check type, flatten namespaces and remove unused code from your JavaScript code files.
The closure library is a library of code you can use in your code and it'll compile with your code http://docs.closure-library.googlecode.com/git/closure_goog_base.js.html (at the moment their site is down though).
There are books that you can read, I found closure the definitive guide by Michael Bolin very helpful. It's an old book but many of the things in the book are still valid.