Red gradient of 21 levels of luminance

Quick answer：

A gradient of 21 levels of luminance in red refers to a gradual transition from a very dark shade of red to a very light shade of red, with 21 distinct steps in between. This type of gradient is often used in graphic design, web development, and digital art to create visual interest and a sense of depth. Each step in the gradient represents a specific luminance value, which can be adjusted to achieve different visual effects.