Tinderbox containers have actions that they perform on notes that are added to (or created inside) the container. Rules are a lot like actions, but they're performed all the time.
Under the hood, Tinderbox keeps a big list of rules it wants to run. Periodically, it runs a bunch of rules. If your machine is slow or busy, Tinderbox will just take a little more time to work through its list.
Rules do some of the same things that agents do. But while agents scan all your notes, rules usually look only at a handful of neighbors. One rule might be, "Turn red if your container is red, but be blue otherwise." Another might be, "If you've got any children with a due date sooner than your own due date, use the child's due date as your own."
I expect rules will be a big help to a bunch of Tinderboxers. Of course, lots of people will rarely or never need them -- and that's fine. Tinderbox is like that! Learning when to use a rule, or an agent, or a container action, might take some practice. Nobody knows the rules yet. Good time to roll up the sleeves!