Focus is sort of like Apple’s take on the good ole AIM away message, but for the modern era. In short, you can set up several profiles for different scenarios that give you more control over how you interact with your phone. On vacation? You can block notifications from your work apps and email. Sneaking out for a workout during lunch? Same thing, except you can opt to allow messages from Slack in case something urgent comes up. You can even create custom home screens that only show you the apps you want to see for a particular scenario.
You can set up Focus profiles by either opening your Settings or swiping from the top right to access the Control Center. Focus profiles are extremely customizable, so you can really go ham experimenting on what works best for you.