The tool generates a stepped gradient between 2 colors.
To change one of the colors, you can use the color picker or preselected swatches. Click on one of the boxes to do it.
To control how many you colors you want to generate, use the slider under the boxes.
You can copy colors in formats: HEX, HSL, RGB. Click on the value, and the color will be copied to your clipboard. If you want to invert the gradient, click on the icon between the boxes.