A Price Rule gives a discount to individuals who have a special tag assigned to them.
You control how much of a discount is given and on what items (ie. classes, products, appointments, etc). A price rule that's just been created basically tells Momence “look for anyone that's buying something, check to see if they have this specific tag and, if so, give them a discount of x amount on these specific items (or all items)”.
Follow this tutorial to learn how to create one in tandem with a tag.