I grew up as a boy doing soldering kits in the 80s. I wanted to do the same for my daughter so bought a soldering iron online. 

It came with a box of orange resin with Chinese writing on it. I'd never seen something like this to do with soldering. Running it through Google translate comes with something like 'solder additive'. 

[![enter image description here][1]][1]

When I google search online for 'solder additive' I get vague descriptions of better joints, or coping better with dusty joints. I don't understand how that would be solved by resin. 

My question is: **What does solder additive do?**

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EDIT: Thanks for all those that contributed to this question and the EE SE community. I asked my Dad what we had done for resin when I was growing up. He responded, "We used resin-core solder, that's why you never saw the resin." 

  [1]: https://i.sstatic.net/Ap4Bm.jpg