The "Bergschrund":
The gap on a mountain where a glacier pulls away from the the mountain itself.  This gap, called a "crevasse", can be quite deep and can pose a danger for climbers.  Falling into a deep crevasse can cause injury or death.

Depending on the snow level, temperatures and amount of sunlight hitting the glacier, the gap between the glacier and the rock can be significant.  At other times, the gap can be easily crossed on foot or via a snow bridge.
 
In the case of Mount Hood, there is a single Bergschrund along the Hogsback (shown below) that can easily be avoided during the main climbing months of May and June.  However, by mid-July, the gap becomes too big to be crossed without special knowledge and equipment.