An overexposed image is technically speaking, an image that has a loss of details in its highlights. The result can be a washed-out image and the brightest area become totally white.
Overexposure happens when your settings in your camera are "off". Maybe your shutter speed is too slow, your ISO is too high, or your aperture is too wide open. Underexposure happens when the opposite happens and the image is too dark.
But, when you think about it, is a personal choice what the correct exposure is. I personally like slightly overexposed.
What are you going to take a photo of today that is overexposed.