A billing threshold is an amount that determines when you'll be charged for your ads based on how much you spend.
You can think of your billing threshold as a way to build up a good payment history with Facebook: It's an amount you can spend on ads before we charge you for them. As you run ads on Facebook, you'll accrue ad costs. Whenever your ad costs reach your billing threshold amount, we'll charge you for that amount.
When you first start advertising on Facebook, your billing threshold will be automatically set to a small amount. But as you make successful billing threshold payments, it may be raised until your account reaches a final threshold amount.
As an example, let's say you just created your first ad and your ad account has a billing threshold of $25. As your ad runs, it accrues costs. If your outstanding ad costs reach $25, we'll charge you $25.
Once your payment goes through, your balance is cleared, your billing threshold may be raised to a new, higher amount and you start accruing costs again as your ad continues to run. So in a given month, you could reach your billing threshold once, multiple times or not at all, depending on what your billing threshold is and how much money you're spending on ads. This is why you may receive multiple charges in a given month. It's also why you may be charged after you purchase ads or even after you've stopped running them.