Smart Wallet is a proxy wallet contract that is owned and managed by your main Ethereum account. In technical terms, Smart Wallet is actually the DSProxy, a proxy wallet used by other DeFi projects, most notably MakerDAO where every CDP/Vault owner has one (meaning you may already have yours ready, too). Using a Smart Wallet is needed because it enables creating and executing complex transactions such as Boost and Repay and, additionally, it includes the option to provide certain privileges to another account, which is what makes Automation possible.