One of the ways of measuring the tonnage of a ship, essentially giving the entire volume of the vessel below the waterline, hence a figure equivalent to the mass of water displaced by the ship's own mass, hence again basically equal to the actual weight1 of the ship.
Displacement tonnage gives an measurement of the absolute size of a ship, without any reference to how the size and space are to be used; as such, it is mainly used for warships, including at various times in the past as a basis for limiting the size of ships under sundry naval treaties; for merchant shipping a variety of gross tonnage calculations of space usable for cargo are generally used instead. The most common displacement figures reported are of the vessel in an empty condition, light of fuel, munitions and other supplies that would always be carried in practice.
1. It is assumed here that we are only dealing with maritime activity on a single planet, since variations in gravity will generally lead to non-optimal performance, so weight=mass can be taken as read.