Assume you have some table in your Drupal database, and you want to fetch some rows from it (directly from the database) to show them on a page.

How can this be done?

To use the Message module (part of the Message Stack), you need to create 1 or more "Message types". But, when using the page to do so (or edit an existing type), near the bottom of it there is a field labeled "Replacement tokens", empty by default, and with some explanation below it like so:

The Message Stack appears to be a set of 3 Drupal contributed modules, that can be used in a Drupal 7 or 8 site to implement messaging in a Drupal site.

Here are some details about it (from that same link):

The modules have been written with a pluggable architecture that supports customization and extensibility. Each module includes implementation examples that give a hint of the potential use cases and integration with other popular Drupal modules.