What is it?
Meteor is a full-stack Open-source web application framework.
What is it like?
I have used Meteor on several projects and found that it was quite powerful and easy to learn.
I liked that it provided the basic building blocks and offered some powerful templating features that would update the UI in real time - a very nice touch.
The testing frameworks and package management tools have matured quite a bit, and there is even tie-ins for Angular and ReactJS built in.