WhatsApp Business differentiates between conversations that are initiated by your business and conversations that are initiated by the customer.
Session messages are the messages you send after a customer reaches out to you and you want to reply. A 24 hour session window opens during which you can freely converse with the customer through the WhatsApp Business channel without being restricted to the use of pre-approved template messages. Every time the customer replies the session window refreshes and it resets for another 24 hours.
If you want to talk to a customer without them having reached out to you, you will need to use a Message Template. Message Templates are pre-approved messages by Facebook that can be sent to customers/users to (re)open a session window; once the customer replies to your Message Template the 24 hour window opens as described above and all messages you send within that time are Session Messages.
Message Templates are usually updates to a reservation or an account, or a different kind of alert. It is not possible to send Message Templates in bulk, and currently Message Templates can only be sent via API in order to initiate the conversation. For best practices on managing your Message Templates, please see this article: Working with WhatsApp Message Templates