The prize pool is at $10 million dollars, and it's averaging 100,000+ viewers tuned in from across the globe for the playoffs. They sold out a 20,000 seat stadium in an hour (in Seattle for the finals). Here's an explainer (think 5 vs 5 people choosing from 100 characters with different roles and abilities, and you have to defend your side of the map), and here is the stream for the uninitiated. It's free to play, but I don't recommend it because the learning curve is insanely steep, and the community is pretty terrible.
Another explainer, but in text.