What are these things, they have weight and they say jar rifle powder adn stuff like that, but i have no idea what they are for.

They are amounts of primer, powder, and cases in convenient packages. You drop them on the ground, then pick them back up (I know it's stupid) and they turn into the respective powders, cases, and primers. These powders, cases, and primers are used to make ammunition.

And funnily, are weightless.

yeah after you have dropped them and picked them back up. if you want for example to make .308 ammunition you need .308 casings , rifle powder , large rifle primer ? and lead , the lead can be made by breaking down bullets or breaking down scrap metal at reloading benches, you also make the bullets at reloading benches.