Geographically, a valley is an area of lower altitude with mountains or hills on either side. Most commonly, they are formed when a river cut down into bedrock, such as with the Grand Canyon. These have a V shape when seen in profile, and can have very steep slopes. When a glacier intrudes into one of these valleys, it causes the valley to widen and take on a U shape. Glacial valleys can be very wide.
Metaphorically, a valley is a time when a soul is going through tribulation, turmoil, depression or just a really bad day.