A snow bridge is an arc across a crevasse, a crack in rock, a creek, or some other opening in terrain. It is typically formed by snow drift, which first creates a cornice, which may gradually grow to reach the other side of the opening.
Dangers
A snow bridge may completely cover the opening and thus presents danger: it creates an illusion of unbroken surface while hiding the opening under a layer of snow of unknown thickness, possibly only several centimetres thick.
Snow bridges may also be formed inside a crevasse making it appear shallow.
Since a snow bridge is thicker and stronger at the edge of a crevasse, a fall through a bridge usually happens at some distance from the crevasse edge.
References
Snow bridge Wikipedia(Text) CC BY-SA