Whatever your payout limit is set at is the minimum amount you need to have earned to get paid. If it is set at $50, then your total earnings need to have exceeded $50 for them to pay you. If you earn $30 this month and $30 next month you will get paid $60 after the second month. If you make $30 this month, $10 the month after, and then $20 the next month, you would not get paid until the third month.
One reason for setting the minimum payout higher is because they charge a fee to process certain payments (international bank transfers I think). If they charge you a fee, then you wouldn't want them to pay you every $20, because the fees would be so high.