A passkey signs you in with Touch ID, Face ID or Windows Hello instead of a code. It's faster than waiting for an email, and there's no code for anyone to intercept or talk you into sharing.
Agents
An agent is an app on your computer, like an AI assistant, that you've allowed to send and open documents as you. Each one adds keys of its own, and removing it removes them too. It can't change your account or billing.