The mixing ratio refers to the amount of developer mixed with the hair color. It will typically appear written as "1:1" which translates to 1 oz of product to 1 oz. (Say your hair color was 4 oz, you would need 4 oz of developer.)
The typical ratio for permanent color will be one part color, one part developer. This is to simply deposit the shade selected. However, adding double the amount of developer will cause the color itself to lighten, so you can control the color this way as well as with the shade name. The mixing ratio will vary by brand, and to figure out the recommended ratio look on the box
for the brand's instructions.