The Dangerous Species Act was an Imperial legislature which decreed that any species that was dangerous and interfered with Imperial occupation of a planet was subject to eradication by the Empire. The act was only intended to apply to non-sentient lifeforms or wild beasts.
In the days following the fall of the Republic and the Jedi Order, many species began to rebel and fight back against the Empire’s occupation of their planet. As a result, Palpatine and the Imperial senate passed a law called “The dangerous species act”, that could classify any species who interfere with imperial occupation of the planet as “hostile”, which would mark them for eradication by the Empire. The law was only supposed to apply to non sentient beings or wild beasts, but Palpatine of course used it against anyone who stood in his way.