Comfort or Homesickness is a biological drive to approach home, in an approximation of the nesting instinct. In C3/DS, comfort builds up in female creatures while pregnant. They instinctively seek out the appropriate home smell when pregnant to lay their eggs in a 'safe' place - unfortunately, this location often doesn't have much in the way of food or toys. Comfort is not seen in the HoverDoc, and is reduced by reaching the peak of home smell and by laying an egg successfully.
Male creatures also experience the comfort drive.
Most creatures in C3/DS (of either gender) also have a stimulus that gives them a large amount of comfort drive when they pick up a creature egg. Grendels will use this to steal off with eggs to their home, because they also have an instinct to pick up eggs, but if you have a norn 'get' an egg, it will also instinctively start heading off towards home... Because they instinctively seek out home while carrying an egg, by rewarding them for picking up eggs, you can teach norns to seek out eggs and bring them back home.