What is the work of guard cells in a plant?

Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. 

They are produced in pairs with a gap between them that forms a stomatal pore and helps to regulate CO2 and the rate of transpiration by opening and closing the stomata.

The stomatal pores are largest when water is freely available and the guard cells are bloated and closed when water availability is critically low and the guard cells become flaccid.