A phrase, I would say, used by those who do not adequately
explain evolutionary theory. (Darwin did use the term, in the 6th edition of his
Origin of species).
The basic thesis of Darwinian evolution is that
random changes can lead to nontrivial adaptations by means of natural selection, a continuous process which preserves those variants that fit in best with the situation at that time and place.
Hence, "survival of the fittest". It is not a tautology.