The battery is probably dying which doesn't necessarily matter if you're always plugged in, though it will (when it totally dies) mean that, for example, you can't unplug and move rooms without restarting the laptop. If you don't mind being always plugged in and it's just the light that's bugging you... stick a bit of insulating tape over it?
.
LT is correct believe it or not though, you should try draining and then recharging the battery a few times to see if that helps, especially if it's an older laptop. Older laptop / phone batteries "liked" to be drained for battery health rather than constantly topped up at a power source. If it's less than a few years old that shouldn't matter though, charging tech has changed so that batteries don't "trickle charge" at 99-100% any more.