Symbiosis is a close relationship between two different species [kinds of living things] in which they both benefit [get something good] from each other. Examples of symbiosis include the relationship between bees and flowers, where the bees help the flowers by spreading pollen [a powder produced by flowers that helps plants make seeds], while the flowers provide nectar for the bees to eat.
Very Simple Definition:
Symbiosis is a special friendship between two different kinds of animals or plants where they both help each other. For example, bees and flowers are friends because bees help the flowers by taking pollen [a powder used to make seeds] from one flower to another, and the flowers give the bees sweet nectar [flower juice] to eat.