The half-life of any chemical, either in the Creatures series or our own universe, is the time it takes for a given amount of any chemical to decay to half the original value. In Creatures, altering this can have profound effects on biochemistry.
In creatures, the half-life of any chemical is determined by a big gene called, oddly enough, the half-life gene. A common mutation of the half-life gene produces a longer half-life for ageing or life, leading to longer-lived creatures.
See Biological half-life on Wikipedia for more on the real-life topic.