From what I can see, this is a bit of galvanised steel pipe squashed at the ends and it is supposed to hold some "magic" substance that cleans your chimney and makes your fire burn more efficiently.
I think they are supposed to contain a bunch of chemicals that react with the creosote and burn it in place. Ie they create an intentional chimmeny fire. I presume they also raise the chimmeny temperature. I would be cautious using this as how would you know if the fire load in the chimmeny is too high to intentionally burn it out? I am sure these are intended as part of a regular maintenance plan and I would want a proper sweep first to be sure I started out with an appropriatly clean flue first.
