Friction can charge insulating materials. When fuel flows quickly through a pipe and nozzle, or when a car body or an aircraft moves through the air, charge can build up on the surface. A large build-up of charge creates a big potential difference between the object and its surroundings, and the charge can jump across the gap as a spark.
Sparks are dangerous where there are flammable things around. Near a fuel tank there are vapours that can catch fire, so a spark when fuelling a car or an aircraft could cause a fire or an explosion. Fine dust, such as flour or grain dust, and some gases in a factory can burn in the same way. A person who builds up charge can also feel an unpleasant shock when they touch a metal object.
The danger is removed by earthing. A metal wire or strap joins the object to the ground. Metal is a conductor, so electrons can flow through it. Any charge that builds up flows safely to or from the ground, so the object stays neutral and no spark is possible. For example, a tanker and the aircraft it is filling are joined to earth by a conducting wire.