A list block is repeating content for a list of items, for example, restaurant reservations linked to a room reservation. The list block is shown once in the template editor where you can add relevant merge fields, and when the email is sent the content will be repeated for each restaurant reservation.
In the image below, this template would list this table for as many restaurant reservations it can find linked to the room reservation.
To create a list block within a template, click on the list block button in the tool bar.
This will show all available list blocks depending on which template type is being created/amended.