It is not a cousin of that Jason in the movie with a chainsaw!
What is it used for?
It is commonly used for transmitting data in web applications (eg. sending some data from the server to the client, so it can be displayed on a web page, or vice versa).
Why is this important to GIS?
GeoJson is an open standard format designed for representing simple geographical features (points, lines and polygons) along with their non spatial attributes.
GeoJson adds the Spatial Component (WHERE) to the Data Component (WHAT).
Chances are when you use your mobile phone to give you directions from A to B the data is being sent to your phone with GeoJson.
For more info, check out these links: