Official documentation for Keychain can be found on the official Keychain wiki page.
Keychain helps you to manage ssh and GPG keys in a convenient and secure
manner. It acts as a frontend to ssh-agent and ssh-add, but allows you
to easily have one long running ssh-agent process per system, rather than
the norm of one ssh-agent per login session.
This dramatically reduces the number of times you need to enter your
passphrase. With keychain, you only need to enter a passphrase once every
time your local machine is rebooted. Keychain also makes it easy for remote
cron jobs to securely "hook in" to a long running ssh-agent process,
allowing your scripts to take advantage of key-based logins.