From Creatures Wikia
A drive is a biological need. Often graphically represented in the Creatures series by a bar, with green at one end (for not needing to do something), yellow in the middle, and red at the other end (indicating an urgent drive). The drive lobe of the brain contains the current state of a creature's drives, and is monitored by the Health Kit and Science Kit in Creatures 1 and Creatures 2.
If a creature has no drives, it does not feel the need to do anything - this is why gengineering your creatures to remove their need for food or making them super-resistant to disease can result in a very boring game!