WhatsApp customer service window
The customer service window is the 24 hours that start when a customer messages your business on the WhatsApp Business Platform; while it is open you can reply with free-form messages, and after it closes you can only send pre-approved template messages.
Each new message from the customer starts a fresh 24 hours.
Why it matters for a small business
It decides what you can send, and when. Prepare answers before you need them.
Source: Meta for Developers