http://aurorawiki.pentarch.org/index.php?title=Terraforming#Colony_Cost_and_TerraformingA world that is perfectly habitable to your people is displayed as colony cost of 0.
This requires an acceptable gravity and atmosphere.
If a planet is within the gravity tolerance of your species then the colony cost displays how much infrastructure is needed to sustain a population on that body due to the hostile atmosphere, colony cost of 1 requires 100 infrastructure for 1 million people, cost of 2 requires 200 for a million, etc.
Reducing colony cost requires removing toxic gasses, changing the temperature to one acceptable to your species, lowering or raising the pressure, and finally ensuring there is the right quantity of oxygen.
If the body is very low gravity then you'll need to use underground infrastructure, which is always calculated as if the body has a colony cost of 3.0, and must be built on site, by the population and initially by construction engineers. Normal infrastructure costs 2 duranium each but underground costs 10 duranium, so it's more expensive and takes a while to build but is the only sensible option for low gravity colonies. Unless you use orbital habitats which have the advantage of being movable.