ssh-agent is a key manager for SSH.

ssh-agent holds your keys and certificates in memory, unencrypted, and ready for use by ssh. 

ssh-agent saves you from typing a passphrase every time you connect to a server. It runs in the background on your system, separately from ssh, and it usually starts up the first time you run ssh after a reboot.

The SSH agent keeps private keys safe because of what it doesn’t do:

• It doesn’t write any key material to disk.
• It doesn’t allow your private keys to be exported.
• Private keys stored in the agent can only be used for one purpose: signing a message.