To be workable, a tile:
- must have nonzero production (peaks, desert, etc. are not workable)
- must be in a city's big fat cross
- must be culturally controlled by the civilization owning the city.
To work a tile, the civilization must place a point of population on it on the city screen. For your own cities, or cities you have full vision on via espionage, you can see where population is working on the city screen: if a tile is being worked, it will have a white circle on it. Players say that they are working a tile.
The primary effect of working a tile is that the city gets the tile's production. However, there are two additional effects: