The Caecum is the enlarged region (in mammalian herbivores anyway) of the large intestine. The function of this pouch is to accommodate microorganisms that help breakdown accumulated plant matter such as cellulose that otherwise cannot be broken down by the host animal itself.
These microorganisms have a specific relationship with the host animal. Such relationships are given a term - symbiosis. There are several forms of symbiosis, but the one we have here is mutualism. This term means that all participants benefit in some way from the interaction. The animal is able to digest components of plant matter due to the actions of the microorganisms and they in turn gain sustaining nutrients from the interaction.
What microorganisms ? Anaerobic and aerobic bacteria (the vast majority both in species and raw numbers of the gut population), fungi, yeasts and some protozoans.
