Yes, it’s a custom smartapp. You install it to your own account, and after that you can open it from inside the SmartThings mobile app to create your own rules, which core calls “pistons.”
CoRE (now WebCore) is a very powerful community – created rules engine for SmartThings. It allows you to set up stacked conditionals like “If A then B but only while C and not if D.”
Setting it up can be quite complex, but there are many community members who will be glad to help you.
For example, here is a piston that one member created to notify him if the dog had already been fed twice that day and he picked up the dog bowl for a third time. He needed this because he has a large family and people didn’t always remember whose turn it was to feed the dog.