The device on a bicycle that warns others of your presence is a bell.
A bicycle bell produces a short ringing sound when its lever or thumb button is operated. Riders use it to alert pedestrians, other cyclists and sometimes drivers that they are approaching, especially on shared paths or in crowded areas. It communicates presence without requiring the rider to shout.
A bell is different from bicycle lights. Lights improve visibility and help others see the bicycle, particularly in darkness, while a bell is an audible warning device. A whistle or horn could also make a warning sound, but a bell is the conventional and widely recognized device fitted to ordinary bicycles.
Bell design is simple: a striker, usually moved by a rotating lever, hits a resonant metal dome. Local traffic rules vary, and some places require bicycles to carry a bell or another audible warning device. Riders should still slow down and leave space; ringing a bell does not replace careful, predictable riding.