tl;dr: What is a keyless blockchain account? Put simply, “Your blockchain account = Your Google account”. In other words, this keyless approach allows you to derive a blockchain account from any of your existing OpenID Connect (OIDC) account (e.g., Google, Apple), rather than from a traditional secret key or mnemonic. There are no long-term secret keys you need to manage. There is also no multi-party computation (MPC) system managing your account for you. As a result, the risk of account loss is (more or less), the risk of losing your Google account.
I wrote a high-level explanation of how keyless accounts work on the Aptos blockchain here.
An accompanying tweetstorm is here too.
Plus, a short presentation here.
For a more in-depth discussion, see the 61th Aptos Improvement Proposal here.
$$ \def\Adv{\mathcal{A}} \def\Badv{\mathcal{B}} \def\vect#1{\mathbf{#1}} $$