This probably means wBTC (wrapped BTC) which is a ERC20 token on the Ethereum Network. They seem to have a more-or-less 1:1 peg to BTC and they seem to be transparent about their holdings (https://wbtc.network/dashboard/audit) - there's a list of addresses that's verifiable on-chain. Disclaimer: I've never used them and the info above comes after 3 minutes of skimming their website ;)