If you are using the GPS when driving, then you are likely plugging it into the auxiliary thingy in the car. You can also plug it in to your computer, as others mentioned. If you don't use the GPS for a while, you may wind up with a low battery and it will take a few minutes to charge. Meanwhile the screen will not be fully lit - your GPS is working fine, and it will brighten up as it acquires its charge.
DA charger comes with the Garmin - one to plug into the car to make it work while driving plus one that attaches to the computer (download the lifetime maps) and I suppose to recharge the GPS if needed, but it charges while using it.