You are on box A and you want to connect to box C (to some port),but there is not direct connectivity from box A to box C on to a particular port: e.g.
A $ telnet C_IP_OR_NAME C_PORT ( does not work)
However, there is a box B which has connectivity to box C to that port
B $ telnet C_IP_OR_NAME C_PORT ( does work)
Assuming you have ssh connectivity from box A to box B, then start port forwarding on box A to be routed to box C via box B:
ssh -f B_USER@B_IP_OR_NAME -L A_PORT:C_IP_OR_NAME:C_PORT -N
that will make every call to the A_PORT on box A to be routed to box C port (C_PORT) via box B