The ssh-agent command is a program that runs in the background and acts as a key manager for SSH. It can store multiple private keys in memory and provide them to SSH clients when needed. This way, the user does not have to enter the passphrase for each key every time they log in to another machine using SSH. The ssh-agent can also forward the authentication request to another agent running on the original machine, allowing the user to hop between different machines without re-entering the passphrase1.
To use ssh-agent, the user needs to start it and add the private keys to it using the ssh-add command. The ssh-add command can also list, delete, and lock the keys stored in the agent. The user can then use the SSH_AUTH_SOCK environment variable to connect to the agent and use the keys for authentication2.
The other commands are not related to the ssh-agent. The sshd command is the SSH server daemon that listens for incoming connections and handles the authentication and encryption. The ssh-keygen command is a tool for generating, managing, and converting SSH keys. The ssh command is the SSH client that initiates the connection to the remote machine3. Reference: 1: SSH Essentials: Working with SSH Servers, Clients, and Keys. 2: [ssh-agent(1) - Linux manual page]. 3: SSH command usage, options, and configuration in Linux/Unix.