What guard cells guard?

They are the pair of cells that surround the stomata (pores on the underside of leaves)

They can change shape to open or close the stomata. Most plants open their stomata during the day (to allow co2 to enter for photosysthis) and close them at night. However cacti open their stomata during the night to reduce water loss, and have to use stored co2 for photosysthesis.