It's a league that is played here in the States during the month of July. Shorter sets, a total of five matches per night, team scoring. It can be kind of fun, it's very noisy, some people think that its not real tennis. The players that I know that played it enjoyed the paycheck, but the schedule can be brutal (3 different cities in 3 days sometimes). If you read around in that website I listed above, you might get more of an idea about it. It's all exhibition, doesn't count for anything really.
Pros should stop wasting money on coaches and doctors and just read the experts on this forum.