This ratio refers to the phenotypic ratio of a Dihybrid cross. Since within a Dihybrid cross we are analyzing both traits, and or Genotypes, for instance the colour of pea plants and whether or not their texture is smooth and or wrinkled we would use a set of letters for each characteristic.
For example:
A = yellow color
a = green color
B = smooth texture
b = wrinkled texture.
The ratios refer to how many plants from a typical number of 16 plants, carry the first 2 dominant traits A and B - 9, carry 1 dominant A and 1 recessive b - 3, carry 1 dominant B and 1 recessive a - 3 and finally 1 recessive a and 1 recessive b - 1.