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