Stash chests are chests that allow you to easily move items from your inventory into a chest. It works by using the first two lines on a sign like this:
[stash]
Stone
That sets up the stash chest. Simply right-click it to have all stone in your inventory transferred to the chest! You may use ID:Data, and IDP will convert it for you.
You may also place other things on the second line. "all" means it will stash all the items in your inventory, including armor. "Armor" will stash only your armor. "Inventory" will stash all the items in your inventory. "hand" will stash the items represented by the item in your hand. "player" will stash the items from the chest into your inventory, and "backpack" will stash all the items from your backpack (goldies) into the chest.
By default, the hotbar is left alone unless you specify a material on the second line, "hand" is placed on the second line, or the third line has "allowhotbar" on it.