A protein isoform is one of a number of different structurally similar proteins that are created as the result of alternative splicing or from similar genes formed from a copied gene and differentiated as the result of evolution.
The creation of protein isoforms is an evolutionary function that allows for a huge diversity of different proteins to be created from a small amount of genetic material and facilitates slight protein modifications.
See the figure below for a simplified representation of how the negation (a form of alternative splicing) can induce the creation of different protein isoforms.
