This SDK lets a Coinbase customer "add crypto" to their Metamask wallet, for example, without first having to use Coinbase's website or app to buy the altcoin (or if that customer already has that altcoin in their Coinbase wallet, they can transfer the altcoin from their wallet's balance instead of buying).
From the blog post announcing the SDK:
Coinbase Pay is the powerful new way to purchase or transfer crypto to a wallet or dapp in a few clicks.
Customers can use their existing crypto balances or purchase new crypto using the payment information saved to their Coinbase accounts, such as debit cards and bank accounts.
From the SDK's FAQ:
Coinbase Pay currently supports the following cryptos and blockchains:
- Ethereum mainnet: ETH and ERC-20 tokens supported by Coinbase
- Solana: SOL
- Avalanche C-Chain: AVAX
- Celo: CGLD
:)