What is a ysalamiri in the novel, Heir to the Empire?

A ysalamiri is a type of alien creature that both the Pallaeon and the Imperials are searching for. The ysalamiri creatures block the Force, causing Force-sensitives and full Jedi to be rendered powerless. Karrde is aware of the power of the ysalamiri and wonders what the Imperials want with them.