Here’s a general explainer on Rules to get you started.
It should be noted that portions of that discussion are slightly outdated or ignore new options, like the powerful API Browser+ that Bruce mentioned.
Rules can be written in YAML and submitted via the SmartThings CLI (command line interface) , or in JSON and submitted to the API. I personally prefer the Postman/JSON route if only because of my familiarity with both from so many past projects, although I admit that YAML can be easier to grasp, especially with more complex rules.
The whole process can look a bit intimidating at first glance, but there are lots of examples and some incredibly knowledgeable people here in the community that can help when needed.