Basically to create your individual BAT account for the Wallet it needs a little bit of SOL to start it. So you need to possess some SOL in your wallet. If you don’t have it, you need to buy it or have it sent.
It won’t use much of it, but the wallet will need to see at least one dollar worth of SOL in the account.
SOL is the “gas” token used to move anything on Solana. So when you receive your BAT and might want to move or withdraw it somewhere, it will use SOL to complete the transaction.