A bell is a hollow object, usually made of metal, that makes a deep ringing sound when it's hit or shaken. In the old days, the sound of a dinner bell meant that it was time to eat.
A belly dancer might have tiny bells on her costume that ring as she moves, and the church in your neighborhood might have a huge bell that's rung every hour. Other kinds of bells include a door bell, a bicycle bell, or a school bell, all of which make a sound to signal something. The Old English root is belle, from a Proto-Indo-European source, bhel-, "to sound" or "to roar."