The primary theme of the book Frindle is that words have power. Nick asks his teacher, Mrs. Granger, who invents words. He asks who decided that the word "dog" referred to the pet. Mrs. Granger tells him that she, him, and "'everyone in this class and this school and this town and this state and this country'" are the ones who decide what words mean. She tells him that people "'decide what goes in that [dictionary].'"
With this information in mind, Nick invents a new word, frindle. This word describes a pen. He and his classmates start calling "pens" "frindles," much to the annoyance of their teacher. The word becomes more and more popular. Soon all the kids at school are using the word frindle. Nick becomes famous. He goes on the radio and television to talk about frindles. There is frindle merchandise and a licensing deal. Nick becomes wealthy from frindles.
Nick proves that words have power. He starts with a simple word, but in the end he becomes a person of influence. He becomes wealthy and powerful because of one word.