You can reuse OpenSSH connections to the same server when you want to open subsequent connections to the same server. For example, if I ran ssh vivek@server42.cyberciti.biz, the ssh client will establish a new SSH connection. It usually takes only a few seconds. However, if I run ssh vivek@server42.cyberciti.biz command again, connecting to a server42.cyberciti.biz multiple times result into the overhead.

This tutorial talks about ssh features called multiplexing that reuse an already-established connection when creating a new SSH session.