It's a ferrule. It's a separate piece from the tubing and then when you tighten it down it squeezes it on there so it seals by compression when it's compressed. As long as it's not too squashed or too loose it's probably fine.
Yes, get it very clean, and tighten it just enough it doesn't leak.
If the other fitting can be counterheld while tightening the pictured part to it, do so.