Fertility
Fertility is the measure of how fertile a tile is. In other words, it's the healthiness of the soil. The closer a tile is to naturally-placed water, the more fertile, grassy and green it will be and look. Also, the higher the fertility, the more fauna can sustain itself there. It also controls how well farms yield crops, so players should always build farms on the most fertile land they can find.
There is no way to improve the fertility of any land. Even using the Dig Canal command to create more water will not improve fertility (but will still improve the irrigation factor for farms). Only water placed by the game improves fertility. However, there is a way to make use of high-fertility tiles covered by water. Simply use the Remove Water command, and the tile will retain its high fertility.