It means that you take over the control of TCP connection.
TCP is a generic transport protocol, whereas HTTP is an application
protocol on top of TCP. The OSI seven layer model
describes TCP as layer 4 and HTTP is layer 7.
If you need to implement a different application protocol, this is one
use-case for hijacking.
Or if you need to do something specialised with HTTP, like preventing
keep-alive connections, that is another use-case.
An example for an alternative web application protocol is Google's SPDY. It's also a
good reason why you might hijack an existing HTTP connection, rather than
create a TCP connection directly. For SPDY, a browser would first make an
HTTP request that included 'accept' headers indicating that it is also able
to understand SPDY. So now you could hijack the connection and implement
SPDY instead of HTTP.