In Despotism, any tile that produces more than 2 food/shields/commerce gets a -1 pentalty. That means that a tile that normally would produce 3 food would only produce 2. A tile that produces 4 food would only produce 3. Of course, tiles that produce 2 and 1 food would still produce 2 and 1 food respectively.
EDIT: There's a help file that explains all this, but it appears to be slightly inaccurate for the tile penalty (everything else seems fine as far as I can tell).