It means that electric charge exists in integral multiples of an elementary unit of charge rather taking on continuous values.
Since Millikan was the first to measure the value of the electronic charge in his famous "oil drop" experiment, he may be the one given credit for discovering quantization of charge.
If the pebble sinks it has a greater density an the twig has a lighter density just like water and oil. water sits on top of oil because of their density
