A base has citizens in it, which can be used to work any tile within range of that base. Tiles can have minerals, energy, or nutrients on them. You can also work tiles by use of a supply crawler. When selecting a base you should determine what resources are there you wish to harvest with it, as well as if you can defend it. Rocky terrain can offer you greater minerals, but is less productive in nutrients even if you build a farm there. Some tiles will have a symbol on them showing a bonus in minerals or energy.