This is the minimum rate you accept for one popunder. For example if you enter 0.001, it means that your rate will not be lower than $1 for 1000 popunders. Remember that the higher you set this, the less popunders will be shown. If it is set too high, some countries might not be getting popunders at all limiting your revenue(for example if you set it to $2.00, you are not likely to get any revenue from India traffic).