There are countless explanations you can read online about why people see the dress as two completely different colors.

In simplest terms, it all has to do with how your brain processes color. Basically, light bounces off objects in the world and reaches your eyes in "a mix of wavelengths," which your brain then interprets as color.

As Slate's Pascal Wallisch explained, "this mix depends on two things: the color of the object and the color of the light source. [...] To achieve what color vision scientists call 'color constancy,' the brain calculates color-corrections for an image on the fly. It takes note of the illuminating light and tries to figure out how it might be affecting the color of an object."

Since the photo of the dress was taken in poor lighting with a bluish tint, your brain either sees the dress in shadows (and color-corrects the dress to be white and gold) or in "a fair amount of illumination" (and perceives the dress as blue and black).