A sidebar is a small section of text next to a longer article. A sidebar usually has information that relates to the main story beside it. Sidebar alert: it also means changing the subject in the middle of a conversation.
Sidebars began in the printed newspaper world, and they've moved, along with newspapers and other news sources, to the internet. Most professionally-designed web pages include sidebars. In the legal world, a sidebar has a completely different meaning—it's a courtroom conversation between lawyers and the judge that jury members can't hear. The newspaper meaning dates from the mid-20th century.