Plain English Meaning
A small marker the pilot slides to the speed they want to fly, so they can quickly see if they are on speed without reading numbers.
Full Definition
A movable marker on the airspeed tape or airspeed indicator that the pilot sets to a chosen reference speed, such as a target approach speed, climb speed, or maneuvering speed. The bug provides a quick visual reference so the pilot can see at a glance whether current airspeed is above, below, or matching the selected target.
Context Anchor
Seen on a glass-cockpit airspeed tape, and on some round airspeed indicators, when setting or watching a target speed.