To add to Ranger's explanation, once you have conquered it, you get a bost to production of a certain good. Rock in the example picture would give a boost to your production of Stone. You can find these Goods Buildings in your Research tree, and in your building menu
It's the icon that looks like 3 bricks.
With the bonus you make 5 items / 4 hours, instead of 1. So it's a large bonus and most people focus only on making things they have the bonus for. You only get 2 bonuses / Age, and there are a total of 5 Goods / Age, so you will always miss 3 of them.
You should check out the Guides and FAQ sections for more info on nearly everything about the game
~Mutzena~