"SSH tunnels are used between an ssh client and a
server so that there is one parallel trusted (encrypted) connection
using SSH as its transport.
"When the tunnel is set up, there will be a passive listening
side and one active connecting side. On the passive end we set up a
port so that the tunnel waits for connections of clients to this
port. When a client connects to this port, on the active side there
is another connection to (potentially) another host/port and so the
tunnel connects the client that used the passive port to the new
connection on the active side."