What is the meaning of a marsupial eating its feces?

If you're referring to the family Pseudocheiridae: All herbivores need some way of obtaining as much nutriment as possible because leaves have little nutriment. The pseudocheirids gain extra nutriment by eating their feces, causing any nutriment that had been passed over the first time through to be absorbed into the bloodstream.

If you're referring to koalas: Koalas eat eucalyptus leaves, which are poisonous. In order to digest eucalyptus leaves without absorbing the poison, a koala needs certain microbes in its digestive system. But a koala is not born with these microbes. So when a mother koala is weaning the baby, she regurgitates the contents of her cecum back up to her mouth, and she gives them (the contents of her cecum) to the baby. Then, the baby can gain these necessary microbes.