These can be one of two things, a "Dessicant pack" made to absorb water.
Typically this is food grade Silica Gel, essentially the same stuff used in
those super absorbent baby diapers.
There are also another type that are used that are primarily intended to absorb oxygen. These contain Iron powder and salt, sometimed with a little sawdust.
The same mixture in different proportions are used in disposable hand warmers.
Wanna make one of those hand warmers really work? Exhale through it a few times,
a trace of water can really get them going....
the same chemistry is used in the warming packs for US issue MRE entre'
Those have no sawdust in them and you dump water into them and they
will BOIL the water added from the resulting chemical reaction.