Never fully explain but we can assummed that it is a way to travel faster the regular warp.
Here is wikipedia answer:
The term transwarp has been used a number of times, referring to an advanced form of warp drive used by the Borg. However, the term also refers to a Starfleet development project in Star Trek III: The Search for Spock. Episodes of The Next Generation and Voyager seem to indicate that transwarp technology involves a wormhole conduit directly through subspace as opposed to warping normal space via a manipulation of subspace. However, in the Voyager episode "Distant Origin," a species known as the Voth used a transwarp technology that didn't appear to be similar to Borg transwarp, but rather an enhanced warp technology. It would seem more appropriate, therefore, for the term "transwarp" to refer to any propulsion system that can be considered superior in potential velocity to standard warp drive, without implying any specific technique correlating to this superior velocity.