Seerow's Kindness is a law that forbids any Andalite from giving technology or revealing secrets to other alien races. It is the result of an incident where an Andalite Prince name Seerow gave technology to the Yeerks, who then used it to invade many planets throughout the galaxy and enslaved their civilizations.
Ironically, it was Seerow's Kindness that both started and ended the war against the yeerks. Upon recieveing Andalite spacefaring technology, the yeerks set out to find hosts to enslave so they wouldn't have to live miserable lives as small, blind and deaf but also sentient worms. However, once given the ability to morph, they could become something else entirely, thus relieving them of any need for a host and ending their interstellar conquest.