The quality value means the level of the image quality of a WebP image using lossy compression. Lowering the value reduces the file size but the image quality also gets poorer. Please visit the following link to learn about the relation between the quality value and the image quality.
It is hard to suggest a fixed quality value guaranteeing both the small file size and high image quality at the same time. You need to consider the content of an image and adjust the quality value on your own to find out the most ideal value, but a value between 50-80 is moderate in most cases.
If the quality value is 80 or more, the image quality hardly improves and only the file size becomes bigger. If the value is 50 or less, the file size hardly becomes smaller and only the image quality gets severely damaged.
If you use lossless compression when saving an image as a WebP file, the file has the same image quality as that of the original image and there is no need to set the quality value. Instead, the decrease in the file size is not very big.
You need to choose the best quality value depending on the content of the image; please visit the following link to learn about the know-how.
