Plants are made up of 90-95% water, and water plays many roles in the physiology and chemistry of plants. Water is taken up by the roots and released by the leaves in a process called transpiration. Plants provide structural support for their non woody parts by using water to generate an internal pressure called turgor pressure. When water is scarce during, for example, a drought, turgor pressure diminishes, the plant loses rigidity, and its leaves and stems collapse. This process is called wilting. If wilting goes on for too long, a plant can die.
In the left panel of the image below, my coleus plant experienced wilting on a very hot day. Coleus plants are not particularly drought tolerant, and being on a pot and not watered adequately made it worse. However, within one hour of watering, the coleus plant regained its health (right panel).
Whereas the movement of the leaves and stems during the process of wilting and the recovery phase are slow, plants can also change the turgor pressure of some of its parts very fast achieving rapid movements such as it the case of the Touch Me Not plant and the Venus flytrap.