This sets the number of server alive messages that will be sent. Combined with ServerAliveInterval, this means that the route to the server can vanish for 11 minutes before the client will forcibly disconnect. Note that in many environments, the system’s TCP timeout will be reached before this.
Counterintuitively, setting this results in fewer disconnections from your host, as transient TCP problems can self-repair in ways that fly below SSH’s radar. You may not want to apply this to scripts that work via SSH, as “parts of the SSH tunnel going non-responsive” may work in ways you neither want nor expect!