It is like a lending club between friends without paying interest. Just make sure that you know the people participating in it and how responsible they are when in come to paying money back.
If you are in it make sure that your number is the lowest as possible 1,2,3. If you can not get a low number 1,2 3 be very cautious because there are people that after they get their number, they do not want to continue paying.
Its a savings plan amongst friends, coworkers. Most often the first person to receive the money id the most needed, but it can be discussed who gets what when. Its a good way to get a nice amount of cash for expenses all at once while paying a much smaller amount. No interest are paid.