Genetic Engineering is a process where an organism (or in this case, a human) has select genes and parts of their DNA altered, added to, or replaced. It can be used to make an organism stronger or better suited to certain conditions. It can also be assumed that it has enhanced their physical condition and attributes.
Genetic Engineering & Bionics
Genetic Engineering is not needed for a person to be implanted with bionics. Genetic Engineering helps make the bionics work without scarring, and makes the person more suitable for bionics. Victor Krane, Leo and users of Liquid Bionics are examples of people who have bionics, but are not genetically engineered. Adam, Bree and Chase were genetically engineered by Douglas and possibly Donald, as they were working together at the time. This has made them more compatible with the bionics. However, any human with bionics, genetically engineered or not, will experience glitches due to their nervous systems being only partially compatible with them. Victor Krane seemed to counter this by implanting much of his body with bionic hardware, not just a bionic chip, for more stability.
In Rats on a Train, Donald says that thanks to genetic engineering, Adam, Bree and Chase are more suited to handle extreme climates.