Similar to what Sid mentioned, it’s basically the Chrome dev console but running in the context of the Meteor server rather than the browser.
In the Meteor shell, you don’t have to worry about whether or not you have permission to update a collection, or whether or not you are subscribed to the data you want to inspect (since you have direct access to the db). You can run functions that only exist on the server. You can run server side CPU profiles and look at interesting stuff like
Meteor.server.sessions. You can change global variables (or even overwrite functions like Reval does) to change the behavior of your code without restarting the server.
And, with a little bit of magic, you can run Meteor shell in production and then life is awesome So you can do lots of stuff!