Each container on a ship is secured to the one below (or the deck) using devices called twist locks. The second tier is further secured using ‘lashing rods’ attached to the deck. The stacking of containers needs to be carefully planned, taking into account the ship design and the cargo weight. In this way containers can be safely stacked 10 or more high.
(https://bit.ly/3t4Ne5Q, https://bit.ly/3uvoTGs)