"Weapon upgrade?"

Whats that? Something u can use on your weapon? How u get it?

A weapon upgrade is pretty much what it says it is... its an upgrade for your weapon. Some upgrades give you extra health, armor or even damage percentages... There are many more... You can get them by buying them from people or salvaging from purple or gold items (most of the time)

hope thats a good enough answer

There is a base item, such as a long sword. Basically, a prefix and suffix can be added to the weapon, otherwise known as a sword hilt and sword pommel. You will know when your weapon has an upgrade because it will say X longsword of X. For example, icy longsword of defense. You can salvage the upgrades out, but it will not be a 100% guarantee to get them.

Will u need the expert Salvage kit for that?

Yes, always. You can also buy most weapon upgrades very cheap from other players.