I bought this Asus K53E laptop two years ago but I don't even know what this light means.
I'm talking about the middle one (next to the battety \ wireless)
It represents your laptop's hard drive. Whenever your hard disk is being accessed (reading or writing), the LED will light up.
A cylindrical shape is the universally accepted icon used to represent a hard disk. For background information, there is a good question on SU already.