An easy way to think of cytokines is as a messenger. They carry messages of where the immune system needs to do battle, which mobilises the white blood cells.
The scientific explanation is a group of molecules or proteins that are released into circulation by cells in the immune system, such as T-cells, to act as chemical messengers in our body. The cytokines that are released help to initiate the action of other cells and play a major role in regulating inflammation and immune responses to stimuli. Examples include Interferons and Interleukins.