What moves in and out of the leaves through the stomata?

Stomata are small pores (holes) in the epidermis that allow gases to diffuse in and out of the leaf. Stomata are usually in the lower epidermis, but some plants like water lilies have them in the upper epidermis.

In sunlight:

Carbon dioxide diffuses in for photosynthesis.

Oxygen made in photosynthesis diffuses out.

Water vapour diffuses out.