There are multiple codons which code for a single amino acid. Threonine, for example is coded for by ACU, ACC, ACA, and ACG. You can see that the third position can freely mutate without a resultant change in phenotype. This is called a neutral mutation. If the base in the second position mutates from a C to a G, however, it makes a huge difference if there were neutral mutations present. An original triplet of ACU neutrally mutated to ACC and then selectively mutated to AGC would code for a Serine. An original triplet of ACU neutrally mutated to ACG and then selectively mutated to AGG would code for an Arginine. So a neutral mutation ends up getting "fixed" by selection of another mutation.
Oh, I see what you're asking. Such an individual would proliferate by virtue of other selectable mutations. A sort of piggyback for the neutral mutation. Until it's "fixed", and at that time the mutations do become expressed in phenotype and come under selection.