What does 18% grey mean? What is the grey card?

This is the color gradient standard of black and white. Gray 100% is black, gray 1% is white, and gray 18% is a neutral color between black and white. This color is regarded as the standard color in photography, which means that the exposure of this color in the light environment is equal to the average exposure of various colors in the light environment, that is, the accurate exposure. The gray card is a color card with the color of 18% gray. In the process of photography, you can get a relatively accurate exposure by putting this card in a light environment and then aiming the metering point at the card for metering. The substitutes with the same chromaticity as the grey card are red brick, cement floor, cyan plant and so on.