What do you want to be whenyou grow up? There are many important jobs that grownups do to keep our world running smoothly. Whether doctors, teachers, builders, engineers, farmers, etc, everyone learns a specific skill which they can then use to help everyone else.
In singular-celled lifeforms, one cell has the ability to perform all the functions necessary to keep it alive. Multi-cellular lifeforms consist of thousands, millions, and even billions of cells. The cells of a multi-cellular lifeform do not have the ability to live on their own. They depend on one another to do different jobs that together keep all the cells alive.
Just like people, cells specialize in important jobs. Some cells work to digest food for the rest of the body, others attack diseases, and others, like blood cells, specialize in carrying things throughout the body.