Handlebars are the bar at the front of a bicycle or a motorcycle. You hold them and turn them, and that turns the front wheel, so you go left or right. They usually have two sides with grips, and often brakes and a bell, so English nearly always says handlebars, not handlebar. On a bicycle they sit above the front wheel; on a motorcycle they sit in front of you and are often wider. People also talk about a handlebar moustache, which curls out like the ends of old bicycle bars. That is only about the shape.