You are at hostA, have ssh access to hostB and want to connect to hostC. Unfortunately, hostC only accepts connections from hostB.
Write the following in .ssh/config
Host hostC
ProxyCommand ssh userB@hostB -W %h:%p
From now on, any ssh connection from hostA to hostC will be done using hostB as a proxy. So now
hostA$ ssh userC@hostC
just works.
Seen here.