well first, go back and shorten your link that is screwing up the board.
But fittest when talking about biology simply refers to living long enough to reproduce. Just being good enough to get past the filter of Natural Selection.
The critters that do not live long enough to reproduce do not pass on their genes. Those who do are fit.
There is also the issue of populations as opposed to individuals. Populations that do not live long enough to reproduce die out and become extinct.
But fitness has nothing to do with anything other than being just barely good enough. A characteristic that does not prevent a critter or species from living long enough to reproduce can be carried forward. And when faced with some different Natural Selection filter, that characteristic might be beneficial or harmful.