Terrabioforming is a combination of changing a world (a planet, moon or other celestial body) to make it more habitable, and changing humanity to make it capable of living on the new world. Essentially, it is a compromise between terraforming and bioforming.
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The rationale behind terrabioforming is to circumvent the limitations with pure terraforming or pure bioforming.
Some worlds would be impractical or even impossibly to terraform. Mars, for example, has low gravity, high heavy metal content in the soil, and a low temperature. Terraforming these sorts of unsuitable worlds would be extremely expensive and take a very long time (e.g. thousands of years).
At the other end of the spectrum, bioforming is limited by the inherent limitations of biology. While there are many extremophiles (organisms capable of living in extreme conditions), and some of these are complex animals (e.g. anaerobic loriciferans and hyperthermophilic Pompeji worms), it would be difficult to make an animal that's both intelligent and an extremophile. A brain with high intelligence, such as a human's, requires considerable energy. So while it might be possible to genetically engineer intelligent (post)humans adapted to high pressure, total darkness and underwater breathing, capable of living in the deep sea, the scarcity of food there would limit where they could survive and their population capacity (though some would be able to live near the richest hydrothermal vents).
A terrabioforming process would most likely involve increasing the productivity of a world, and thus the amount of food it can produce. This would allow extremophilic humans to survive, though it would still be lethal to unmodified humans. For example, terrabioforming Mars would involve warming it up, thickening the atmosphere and bringing more water to the surface, then engineering humans to survive at high CO2 levels, high radiation levels and low oxygen levels. This would be much faster than classic terraforming, likely taking decades rather than millennia.