These can serve multiple purposes, but are most likely to be used as emergency shelter. This is a 80 x 96 inch bright yellow plastic tube that can be configured in numerous ways for individual shelter. It comes in a zipper lock plastic bag with printed survival instructions and a book of matches. I ditch the rest and vacuum pack the Storm Shelter, which then fits easily into even a small pocket. Another alternative is to carry a Space Emergency Blanket, but they have their own drawbacks .
Some pilots prefer to carry even more equipment in a waist pouch or survival vest.
Survival vests are available in many configurations. Many are adaptations of fishing or photographer's vests and some pilots just use these style vests in lieu of the real thing. Some work better than others. If at all possible, try it on beforehand, in the plane you'll be flying. For comfort, safety and ease of egress the vest should not be too long, stopping at or above the waist, and not too loose fitting, to prevent it from catching on something on your way out.