A snow layer melted by radiation from the sun and subsequently refrozen.
A frozen sun crust sometimes forms a hard bed surface for future avalanches to run upon, but just as often does not. When new snow falls on a sun crust, it may produce loose snow avalanches and but this avalanche activity is short-lived. Over time through complex processes of heat and vapor transfer, small facets can form near a crust. These facets can become the weak layer for future slab avalanches.
Sun crusts, of course, form only on sunny slopes and not at all on the shady ones. So we find them mostly on southeast, south, southwest and west facing slopes at mid latitudes in the Northern Hemisphere (and conversely forms more uniformly on all aspects in tropical and arctic latitudes). On these aspects many sun crusts can form during a season. Many do not become an avalanche concern while some do.
When new snow falls on a sun crust, it's important to check out whether the sun crust is wet or frozen when the snow starts. If it's wet, the new snow will stick to it and you most likely won't have any immediate avalanche problem, but if the crust is frozen, then the new snow does not tend to bond very well.
Sun Crust Summary:
By strong sun on the snow surface.
Shiny with slightly rough surface.
Forms only on sunny aspects, none on shady aspects - moderately elevation dependent.