What does This Billing Period mean on iPhone? It says it right below what apps use Cellular data

What does This Billing Period mean on iPhone? It says it right below what apps use Cellular data

Your cell phone service provider doesn't bill you for usage from the first day of the month to the last day. Instead, they group all their customers into about 30 separate billing groups each with a range of about 30 days but not all starting on the same day.

Your billing period could be the 1st of the month through the 30th, or it could be the 10th of one month through the 10th of the next, or the 20th to the 20th, whatever.