It's an extension for interacting with dApps and holding funds from inside the browser, functioning on StarkNet, a ZK roll-up at the top of ETH (layer-2). The Braavos extension generates a bridge to each website so that the decentralized apps can connect with the wallet. When the dApp needs to execute a transaction via Braavos Wallet, the user receives a safe interface to check the transaction's self-explanatory before rejecting or accepting it. Braavos needs the approval to r/w to a webpage to add the desired feature to a standard browser context.