When you editorialize, you offer your opinion when it's not appropriate. A newspaper reporter might editorialize by describing a state's Governor as "a loud mouth."
There are many times when it's perfectly acceptable to express your personal opinion, but whenever it's not okay — especially in journalism — you editorialize. In a news article about farming, a writer might editorialize by stating her personal preference for locally grown food, for example. The word editorialize was coined in 1856, meaning "introduce opinions into factual accounts," from editorial, "written by an editor," and the Latin root editor, "one who puts forth."