A slab can form by the natural settlement or the bonding of snow over time and can become very dense and strong. This is referred to as a hard slab. When a hard slab forms over weak, faceted snow, the ingredients are present for an incredibly destructive avalanche. When these conditions exist, it is best to avoid backcountry travel in all but the tamest areas. Slabs can also form by wind transporting snow to one general area and essentially compacting that snow into one piece.
Wind can produce a very hard slab.
This can occur very quickly and can transform a benign powder slope into an area to be avoided in a matter of minutes. Often wind slab is visible to the eye, but more often, such as when high winds precede a storm, wind slab can be hidden by several inches of new snow, making it very difficult to detect. This is when it might be prudent to allow a little time for the slope to adjust to its new load and seek out an area you know to be sheltered from the wind. Being aware of the prevalent wind direction for the day or days prior will help you find the good, and the safe, skiing. Things change more rapidly than they seem to later in the year.