The most recent version of HDMI, more than doubles the bandwidth of the signaling from 4.95 gigabits per second to 10.2 gigabits per second. This increase in bandwidth enables greater color depths, higher screen resolutions and faster refresh rates. It supports the new xvYCC color space, adds support for the Dolby ® TrueHD and DTS-HD Master Audio standards.
Some people assume that as the signals are digital either the cable works or not. Sometimes however the 1s and 0s aren't all there because of signal degradation due to inferior cable construction. That can be especially true with audio and video sources such as CDs and DVDs. The signal will degrade gracefully, to a point and then it will break up. Music and video is not like data. Digital signal processors can work with a degraded signal and deliver less than perfect sound and pictures.
You can never improve a digital signal by using an expensive cable but you can certainly degrade a signal using an inferior cable.