To keep it simply, after you earn $100, you must wait an entire month. Then they pay you the following month. Like this: earn $100 month (month1) -> wait a month (month2) -> payment this month (month3) Here is a picture taken straight from the Google Adsense payment calendar: Hope this helps.
Yes, I was confused by this at first, but then I looked at it in detail and worked out I'd have to wait two months! Note that this is the maximum time though - you could get checks in just over one month.