Seisan kata is taken from Shorin-ryu, one of the parent styles of Isshin-ryu. The name is sometimes translated as "thirteen," referring perhaps to the number of opponents or the number of moves in the original version of the kata, and sometimes as "three faces or forms," referring to the fact that most of the moves are done in threes.
It emphasizes a toe-to-heel stance (seisan stance or sho zenkutsu dachi) with equal weight distribution and quick, straight-forward fighting techniques.
This is the first of the formal Isshin-ryu kata that students will learn after they have become familiar with basics and the Pinan.