how does potential work in terms of hwo many you can use on a weapon?

so when ppl say they have 2nd line, and going for 3rd line, what are they referring to?

So you can potential as long as you have one available slot? but only potential once? Then how do some ppl have 2 or 3 lines. I assume that using potential scroll successfuly once gives you 1 line ?

Also, if an item came potentialed (from a drop for instance, or crafted), can you potential it again?