Grasslands are wildernesses (wilds) that increase your food resource production if you conquer and own them.
The bonus for owning a grassland is as follows:
Grassland level 1: 5% bonus to food production
Grassland level 2: 10% bonus to food production
Grassland level 3: 15% bonus to food production
Grassland level 4: 20% bonus to food production
Grassland level 5: 25% bonus to food production
Grassland level 6: 30% bonus to food production
Grassland level 7: 35% bonus to food production
Grassland level 8: 40% bonus to food production
Grassland level 9: 45% bonus to food production
Grassland level 10: 50% bonus to food production
As with all wilds, grasslands have defenses that increase in strength according to the level. The number of defending troops per level is the same across all the different types of wilds. Food production can also be boosted by owning rivers.