State of Decay 2's Breach and Clear achievement comes naturally pretty quickly but here's how to do it just in case you don't understand.
When you get into an area like a house, shed, gas station, etc. you'll have a certain number of rooms to clear. This is done by entering them so that the icon on your mini-map disappears. If there are zombies still on the property you'll need to dispose of them too. Doing this in five different properties will get you the achievement. It's something you'll be doing every time you play.