Jin possesses a Quirk that allows him to create a copy of anything, two at a time, through touch. Clones of people possess the same personality and abilities as the original.
However, in order to create a perfect clone, Jin requires a flawless image of whatever he plans to copy. This includes precise measurements and features. The copies are also less durable than the original thing, dissolving into a viscous liquid after suffering enough damage, and a second copy is apparently even more fragile. Jin also appears to be capable of detecting when clones get destroyed if he's not close to them.
Because Jin's clones share the same personality as the original Jin, they can also rebel against him or each other. Having suffered a traumatic experience with this particular trait of his Quirk, Jin refuses to use his ability on himself.