Under watering happens when the soil becomes too dry for too long between waterings.
Signs of under watering include leaf tips browning, leaves dropping, and wilting of plant and leaves.
With most plants it is better to slightly under water than to overwater. When plants are under watered, they can usually recover within a few hours after receiving water. If they are overwatered, this can cause roots to rot, and the recovery process will take much longer.
If the plant leaves are wilting and still soft, they will likely recover after watering.
If the plant leaves are crunchy, they will not recover after watering (this is past the point of recovery).