These are disposable hand and body warmers. A bag about 4" x 5" that contains iron powder, water, salt, activated charcoal and vermiculite.
Shake up the bag and it will keep parts of you warm for a long time, maybe 12 to 18 hours.
We keep some in the house (for power outages) and in our cars and RV. Just in case...........
I have put them in my boots/shoes betwixt sox and insole, when outside temps are cold. I find if I can keep my feet warm, the rest of me feels better.