When a customer contacts you on your WhatsApp Business API account, you are given 24 hours to respond. This is known as the Customer Service Window, and was put in place by WhatsApp to combat spam. During this period, you can communicate freely with your customer, with no limitations on the content or type of your messages. These are considered Session Messages. Every time the customer replies the session window refreshes and it resets for another 24 hours.
After the 24-hour period is over, you can only use Message Templates to contact your customer. If they respond, a new 24-hour window will open, and you will once again be able to send Session Messages. If you want to talk to a customer without them having reached out to you, you will need to use a Template Message as well.
To learn more about Message Templates and how you can use them, check out this article: