Tile yields are determined by a number of factors. (Note that none of this really "matters", what matters in-game is the numbers you see on a single tile. This is more for those interested in what goes on behind the calculation.)
First, they must be the right "environment" - green or fallen. If they are just barren or any of the other types, it doesn't give ANY yields.
Second, the type of terrain that surrounds the tile determines what the yields are. They are gathered from the central tile and all surrounding tiles (effectively a 3x3 grid). Plains give food, forests give materials. Rivers give both.
Third, random spawn of world resources also determine tile yields. Some world resources boost the value of surrounding tiles. Iron ore gives materials, for example. Essence comes from a large variety of world resources - the most common is shards and crystal mines. But they also come from recruitable monster resources.
There are also other factors. For example if there is a nearby city the tile will not produce any yield. A tile must also have at least 2 materials or it will not appear. A tile must also have at least 6 combined material+essence+grain or it will not appear (that may be material+grain =>6 I'm not sure if essence is counted into the combined value).
