Income buildings are your main source of income next to PvP battles. They produce a set amount of Gold Coins in a given time period. You start with the low income Mess Hall as your first source, and by the end of the game every building is unlocked. As you advance in level, different buildings become unlocked, allowing more gold to be produced in a certain time frame.
There are two types of buildings that can be bought with the different currencies in the game.
One set of buildings can be bought with Gold Coins, while the other can be bought with Medals. The buildings bought with Medals producers more coins faster.
The amount of coins per time frame can be changed with a booster being placed near any income building according to the percent that booster adds to a building.
Also note that the higher income buildings allows the opponents to make more loot than the lower income buildings.