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.
Here's an example:
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.
To learn more about Message Templates and how you can use them, check out our article: Message Templates.