An affix is an additional element placed at the beginning or at the ending of a word in order to modify its meaning.
When we analyze the word "polymorphous" we find that the affix - in this case a prefix, because it is placed at the beginning - "poly" is modifying the word "morphous".
The affix "poly" means several, many, or much. When added to "morphous", which means "having the shape", it forms polymorphous, meaning something that can have several different shapes.