To get the Every Piece of Trash in Town achievement in State of Decay 2 do the following:
"Site" refers to anything designated as a location in the game. They come in many shapes and sizes (homes, gas stations, sheds, etc.) and the game's UI will tell you how many rooms need to be searched before each is cleared. Go into every room of 25 sites and this achievement will unlock.
This is also the fastest way to get the following achievement: