Prometheus is configured by a combination of command line flags, and a single configuration file which is commonly called prometheus.yml. You've already seen this in use in previous courses.
Everything about what you want to monitor, how often to scrape, how to find targets, where to load rules from, where to send alerts to and what remote storage systems to use.
It is recommended that you go through this course with the reference documentation open in another tab, in order to better understand how the reference documentation is laid out as you are guided through the configuration file.
This course is targeted at Prometheus 2.0, but most content will also apply to older versions.