A kata is an exercise in martial arts. A code kata is an exercise in programming which helps hone your skills through practice and repetition. You can find out more about code Katas in general here.
The GS Collections Kata is a fun way to help you learn idiomatic GS Collections usage. This particular kata is set up as a series of unit tests which fail. Your task is to make them pass, using GS Collections. The Kata is meant to take approximately one eight-hour day.
Make sure you read the instructor notes in the slides. In addition, you can refer to the GS Collections Reference Guide for further detail and examples of the API.