Lightning arrester

A lightning arrester is a device, basically an air gap between an electric wire and ground, that is used to switch a lightning strike from something such as a television antenna to a metal rod about two meters long that is buried in the ground. If a lightning arrester is not used, then the lightning will go through the television set and burn it out. 
A "lightning arrester" does not really "arrest" or stop lightning. It "diverts" or changes the way lightning goes so that it ends up going into the ground instead of going into a television set or other device that it could hurt.