I read the basic rules, but they mention nothing about the term "overhang". How does the office work, exactly? Is this explained anywhere?
It means that for double-wide tiles; you can place them so that they are only half on your board, with the other half hanging off the board. There is an illustration of it in the appendix in the rules. Basically, it allows you to fit more things on your player board.
Note that there is a minor error in that illustration; as you cannot have a double pasture hanging off the board (since building a double fence requires being built on top of 2 adjacent meadows). Details about that can be found here.