HTTP Tunneling is the mechanism of wrapping data / data stream of other protocols of TCP/IP family as HTTP packets for sending and receiving in restricted networks.
For example, you want to send real-time audio from one machine in your local network to another machine in external network (say internet). And, your local network has firewall restrictions which blocks non-HTTP protocols. In this scenario, both your machine and the target machine wraps the audio stream in TCP to HTTP packet while sending, and then unwraps the TCP packet from HTTP packet on receiving.
This may not be very efficient, but it helps in by-passing some of the restrictions.
For more details: http://en.wikipedia.org/wiki/HTTP_tunnel