What Is This Stuff That Comes With Most New Items?

My son found this in something new he got the other day and asked me what it was? Surprisingly, I couldn't answer his question. lol Is that sad? Or are there others in the same boat? All I know is that you are NOT supposed to eat it! Seriously, this little packet of 'silica gel' is something that I just know to throw away when I get a new pair of shoes or something. lol What exactly is the purpose of this little packet of salt looking stuff? I'll be even more impressed if you know without having to look it up online. lol