A Slayer bell is a piece of Slayer equipment that can be rung to lure molanisks off walls. A Slayer level of 39 is required to operate this item. Slayer bells can be purchased from any Slayer master for 150 coins.
When the bell is rung, the ring is clearly audible to other players. You do not need a Slayer level of 39 just to ring it, but it won't provide any benefits without the required level.