Hills is a terrain feature in Civilization VI that adds +1 
Production Production to a base terrain.
If a city is founded on Hills, the city gains a defensive bonus, making it harder for an emeny to capture it.
Below is a list of all the Hills variants and their yields.
Desert (Hills): +1 Production Production
Grassland (Hills): +2 
Food Food, +1 Production Production
Plains (Hills): +1 Food Food, +2 Production Production
Snow (Hills): +1 Production Production
Tundra (Hills): +1 Food Food, +1 Production Production