What is the number 0.05 in the calculation of contrast ratio between colors?

To calculate contrast ratio the following formula is used (source):

(L1 + 0.05) / (L2 + 0.05)

where:

L1 is the relative luminance of the lighter of the colors, and

L2 is the relative luminance of the darker of the colors.

My question is, what is the number 0.05 in this formula?