Answer:
Guard cells are the two curved cells on either side of the pore (hole). By changing their shape they can open or close the pore.
When the guard cells absorb water they bend outwards, so that the pore between them opens up. When they lose water they go back to a less curved shape, closing the pore between them.
Guard cells bend outwards when they absorb water because the wall next to the pore is thicker than the outer wall, so blah it stretches less.
Usually, stomata open in the day to let carbon dioxide into the leaf for photosynthesis and to let oxygen (made by photosynthesis) out of the leaf. They close at night, when photosynthesis cannot take place, to prevent the escape of water.