Stomata are a plant's main way of controlling water loss from the leaves through transpiration. The stoma, or opening, is bracketed by two guard cells, which are shaped such that when they lose turgor pressure, they press together on their inner sides and close off the stoma.
When you subject plant epidermal tissue to a 10% salt solution, osmosis causes water to leave the plant cells and move toward the hypertonic solution. In most of the cells, these osmotic changes are not visible. Because the guard cells have a free edge, they can change shape dramatically when immersed in hypertonic solution. As the guard cells lose turgor, we see the opening between them being restricted, and we say that the stomate is closing.