There are restrictions on the type of messages you can send. To ensure that there is no misuse, such as bulk messaging or spamming, WhatsApp requires you to get your message templates approved before you are permitted to send them to your contact lists.
There are two types of messages you can send: template-based and non-template-based. The template-based messages are to be submitted to WhatsApp for approval. Once they are approved, you can use them to send messages over and over again.
If your contacts reply to your template-based message, you have a 24-hour window in which you can send a non-template message, such as a reply to a query your customer might have asked. If you miss replying within 24 hours of receiving the message from your contact, you have to use a template-based message to continue the conversation with the contact.