"Frindle" is about a fourth-grade boy named Nick who has a reputation for making life difficult for teachers by doing small things that annoy them. In fifth grade Language Arts, he is more creative than his very strict teacher, Mrs. Granger. She is constantly making the students look up words in the dictionary, and Nick decides that the dictionary needs some revision. So he begins making up new words.
He and his friends make up a new word for a pen - "frindle" - and begin calling it that. Over time, the name becomes very popular in his school, town, and in fact, all over the nation. The adults (Mrs. Granger in particular) try to get him to stop using the word, but by then it's too late. Nick ends up on national television and a local business owner trademarks the word "frindle".
In the end, "frindle" ends up in the dictionary. Nick grows up, goes to college, and finds that he is very rich. He and Mrs. Granger become friends when he finds out that she was just "playing devil's advocate" and was on his side all along.
In the reference links, there's a link to the enotes page that gives a more detailed plot summary. If you're looking for more detail, that would be a great place to start.