Named after the Simpsons character Ned Flanders, Flanderization is a process by which a character's personality is exaggerated over time, usually by focusing on one or two specific qualities and making them more pronounced. In the case of Ned Flanders himself, this involved taking his religious faith and making it more extreme to the point that he became a fundamentalist (though later seasons did somewhat reverse this characterization).
This process is often frowned upon by audiences, who do not like seeing characters become essentially parodies of themselves. However, some cases of Flanderization have actually helped to make a character more memorable and give them a better defined identity and personality, so the process need not necessarily be a bad thing. That being said, the term usually has negative connotations and is often associated with a franchise or work's decline in overall quality.