Terraforming is a (currently hypothetical) process that involves the formation of an atmosphere on a planet by raising the temperature, so that the world develops a biosphere which could support (carbon-based) life. The process increases oxygen level in the atmosphere as the planet becomes habitable for human life. For example, the terraforming process on a world that is covered in ice can be terraformed by increasing the level of greenhouse gases that convert the planet's climate to Earth-like. There are many different processes that transform a planet. Terraforming may take several centuries.
The Gadmeer built a massive ship, which held enough resources to terraform a single planet into a world capable of sustaining their race. Such a world however would be inhospitable to human life. (SG1: "Scorched Earth")