Hey, welcome to the forums!
So, the latest major update in the game brought some big changes to card management concerning the stash and how/when cards are salvaged.
The stash itself is just a 10-card side deck. You can deposit cards in there to save for later or transfer them off to other parties. It was a little more cumbersome before, but players often did this with a "mule" character if they wanted to hold on to a few cards or gear up another party. The stash just cuts out the middle man, and allows all your parties to have shared 10-card space to trade cards around.
Salvaging is a little different compared to what it used to imply. After a scenario, when you "salvage" those extra cards for gold, they are absolutely not lost permanently. You are essentially trading them back to your collection in return for gold. The same way you get a few gold for defeating banes, you now get some gold for acquiring boons by being allowed to sell the "excess" cards back to game. Those cards you "salvage" post-game return to the card pool, and have a chance to being dealt out again in a later scenario.
The "old salvage" where you use to permanently sell off treasure cards is presently gone. Yes, it is confusing that it's the same term being used two different ways, but as of the latest major patch, your actions in the game cannot remove cards from your collection or your account.