A decanter is an container that is usually made of glass and is used to serve wine in. It is a standard shaped bottle but has a glass stopper to seal it. It usually can accommodate the quantity of 0.75 litre to 1 litre bottle of wine within it.
It is similar to another container called a carafe although a carafe does not have a stopper. The use of the stopper is to limit the exposure of the wine to air.
The idea of moving the liquid to a decanter is to separate the clear liquid from the sediment present in the wine.