ssh-agent
SSHPackage: openssh-client
Holds decrypted private keys in memory and signs on ssh's behalf. It does nothing until ssh-add hands it something, and eval "$(ssh-agent -s)" is what makes the shell aware of it.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
ssh-agent -s- Prints the shell commands that export
SSH_AUTH_SOCKandSSH_AGENT_PID. Used witheval, without which the agent starts and nothing can find it.