As guard cells become turgid, the stomata fully and gases and water vapor are to pass through it.

As guard cells become turgid, the stomata fully open and gases and water vapor are allowed to pass through it. When the plant is full of water, the guard cells fill up to become plump and turgid, making the stomata open so gases can be exchanged for photosynthesis. When the plant  is short of water the guard cells lose water and become flaccid, making the stomata close, this helps stop too much water vapor escaping.