Symbiosis is a relationship between two organisms where each of them provides some benefits to the other. In many cases, the symbiotic relationship is so strong that each of them depends on the other for their survival.
The relationship between cows and the bacteria that live in their guts is an example of symbiosis. Cows eat grass and hay. The cows on their own cannot digest what they eat to yield energy for their survival. This is made possible by the presence of bacteria that reside in them. The bacteria are able to convert cellulose and other molecules present in what cows eat to shorter organic compounds that the cows can digest. The cows in return provide the bacteria with a place to live a supply of food.