my wallet finally get synced but unlock button is still off and there's some amount in 'stake'.
I don't know what it is. Can anyone explain it?
The unlock button is still off because your wallet is not encrypted with a passphrase. Once encrypted the wallet will be locked and the button will be on.
You wallet is minting because Coin2 is a Proof of Stake coin. It generates earnings (new coins with a compounded interest) on your incoming transactions regarding the coins age.