It's a pretty simple thing, really, just a four ounce purified water packet for those times when everything else has gone straight to hell. The Datrex packets have a few extra features that recommend them. First, the individual four ounce size is very convenient for both storage and consumption. It's easy to ration out to groups, track how much you're consuming and how much you've got left, and if one gets compromised it doesn't affect the entire supply.
Datrex packets can be frozen, and in fact work nicely as ice packs in that event. Again their small size comes in handy because, if frozen, you can easily thaw one out fairly quickly underneath your armpit, as opposed to say having an entire gallon you needed to thaw. Try sticking that in your crack grampus.
The packets have a five year shelf life, although you should be able to use them well beyond that window, there just may be an odd taste to it. Recommendations for water consumption range from 8oz/day to 64oz/day, but obviously it depends on climate and activity level, to name just two variables. FEMA says that water, unlike food, should never be rationed, and certainly never ration water for children, pregnant women or other vulnerable individuals. Keep a dozen or so – or more if you're in the desert – of these in your car, backpack or wherever along with some water purification tablets and maybe a portable filter and you're good to go.