It means if you deposit 1 BTC to the plug wallet it will be on chain (held in your ownership) then once you transact through the plug wallet it’s converted into ckBTC which you will transact with. Once you want to exit or transfer the ckBTC to your hardware wallet (example) the value of BTC which remains,is transferred into your hardware wallet as BTC ( I believe when transacting within plug wallet or transfer to plug wallet there are no gas fees 👍