HDMI stands for High Definition Multimedia Interface, but what does an HDMI cable do? It transmits digital video and audio signals between compatible devices through only one cable. HDMI-compatibles devices include a huge range of audio visual equipment including source devices (Blu-ray player, DVD player, Playstation, Xbox, set top box, Foxtel box), display devices (TV, projector, computer monitor) and more (AV receivers).
Just as there are many devices which use HDMI cables, there are a variety of lengths which you can choose from. Ideally you will have a cable which is not too short, yet not too long that you are tripping over them, having to tuck the excess in somewhere. Both extremes will contribute to reduced life of your cable. If you are not sure how long you need, use a length of string or ribbon between the connecting devices, cutting it allowing a slight bit extra for the connector heads, which will be inserted into the devices.
HDMI cables are like batteries and birthday candles. It never hurts to have a few spares in alternate lengths about for when you reconfigure your furniture arrangements or if your cable reaches its end of life. It can be incredibly frustrating when something so simple stops you from enjoying your electronics.