A TEU is a 20-foot container, or the space it occupies. A 40-foot container, standard or high cube, counts as 2 TEU. It is the unit used to compare a ship’s capacity, a port’s traffic or a line’s fleet.
For the forwarder it is a statistical unit, not a pricing one: freight is quoted per 20’ or 40’ container, not per TEU. When a port announces millions of TEU, it is counting container moves, not shipments.