A pool ship is a Yeerk "mother ship" containg a large Kandrona pool. It resembles a black spider, with a fat spherical body and a flattened bottom, three leg-like engines, and several small tendrils on the bottom that act as sensors.
Kandrona pool, is a place were the yeerks can swim around until they reach earth.
Near the end of the war, after the Animorphs had destroyed the Yeerk pool, Visser Three, who had recently promoted to the rank of Visser One), temporarily transferred his headquarters onto the Pool ship and ordered it to land on Earth. The visser also ordered the complete obliteration of the Animorphs' hometown in order to create a sort of dead zone around the ship. The ship was to provide nourishment for ground-based Yeerks until a new pool could be created. The Animorphs captured the Pool ship and Visser One, with the help of the military, the Auxiliary Animorphs, Toby's Hork-Bajir, a group of Taxxons led by Arbron, and a split faction of Yeerks led by Tom who wanted to create their own Empire.