in the meaning of a polymorphic gene:
A gene can have more than one specific DNA sequence. For instance, everybody has a gene for albumin (which in mammals is the most abundant protein in blood), but the precise sequence of bases or nucleotides might vary. This can have three possible outcomes:
First a silent mutation. Here there is no effect on the amino acid chain of the protein (silent polymorphism) it encodes, and therefore have no effect on the proteins function.
Second an amino acid might change into a different amino acid with chemical and/or spacial characteristics that are so similar to the "original one" that the protein is unaffected.
Third an amino acid can change, affecting the proteins efficiency or specificity. This is the kind of polymorphism that would influence the hosts biological fitness, and therefore drive evolution.
Theoreticly a fourth option is possible, a change could occur that would introduce a stopcodon or a frameshift (usualy this is caused by an insertion or deletion of a number of bases not dividable by 3) that would make it impossible to read the correct protein. This event is techninaly a gene polymorphism, but generaly termed a nonsense or missense mutation.