What does "1st charge will be for…" mean in the billing policy? (Essential Subscriptions app)

Under each subscription option's Billing policy, the line 1st charge will be for… shows the period the customer pays for at checkout. It follows your Bill every setting.

• Pay as you go plans bill as often as they deliver, so the first charge covers one delivery period.
• Prepaid plans bill for several deliveries at once, so the first charge covers the whole billing period.

It isn't a free trial or a delayed first charge. The customer pays at checkout. See Prepaid and Pay as You Go Subscriptions Explained.