Wired trasnmission media is any medium used to transfer data physically over a wired network. There are various kinds of cables that can be used for this.
Wired data transfer cannot be intercepted like wireless, and so at first glance, this option seems more secure. However, it is possible to place a scanner on a network cable to scan for signals. The way this works is that it is constantly and rapidly scanning for the pulses of electricity that come from binary signals, and uses this to assemble unencrypted data.
Some types of cable are designed to be secure against these kinds of attack, category 6 (Cat6) cables have an internal metal coating which alters the electromagnetic field so that scanners cannot pick it up. Cat5 cables on the other hand do not have this feature and are liable the scanning.
It is difficult to steal information using this method, because placing a scanner would require the attacker to physically reach the target network, but this kind of attack is very dangerous because it can replicate secretive data while in transfer without the network users knowing.