It’s actually quite simple. At its most basic, the feed rules work as the following: if X, then Y. That’s still a bit vague though. If an attribute of your product has the value ‘X’, then your rule applies to it.
For instance, let’s say Black Friday’s coming up and you want to add “Black Friday Sale” to the product title of only products with a specific brand name. You would instruct the Merchant Center to apply your new rule to all products with the value “Example Brand Name” in the brand attribute.
You then choose what you want to happen to all of these selected products, in this case prepending or appending “Black Friday Sale” to the product title. Once you’ve applied your new rule, your product titles will reflect your change.
Additional modifications can also be applied to your feed rule, making changes to your inventory much smoother.