Silver Stackers actually has two meanings.
The first meaning refers to a specific type of silver bullion. Made by the Scottsdale Mint, this unique form of Silver bullion come in three sizes. 2 ounce, 5 ounce and 100 g silver rounds. They feature a lion wearing a crown (The symbol of the Scottsdale Mint) and interlock together regardless of the size of the rounds being stacked.
The second meeting refers to people who invest in silver. There are many forums dedicated to “silver stackers”