Not stacked, but staked. It is a way the coins can generate interest - theoretically the interest can be 5% per year, but due to stake blocks having own difficulty and competition (there are many wallets wishing to stake coins) the actual interest is somewhat lower.

Coins need to be at lest 7 days old (i.e. you need to have them in your wallet for at least 7 days, the counter is set to zero when you the coins are sent to new address) to have a chance to create staked coins, the older the coins, the more stake you can earn, but after being 30 days old the stake value does not increase (i.e. more than 30 days old counts as 30 days old)

Once the stake happens, some part of your coins (let's say it will be 50 CACH) will turn into "mined by stake" and will increase their value. So instead of 50 coins that can be just sent as payment you will suddenly have between 50.05 - 50.21 coins based on coin age - but these will need 520 confirmation before you can manipulate them (either sending them or hoping for another stake from them)