Phase angle of the light radiation.
“k” is often used for a particular constant, usually with a clarifying subscript. In this case we can use dimensional analysis to at least see what form it takes. Lambda “λ” is usually wavelength in this context, and ‘r’ would be a radius to define an circumference (2πr).
So if we use SI units with meters for length, we have a circumference divided by a wavelength will equal our ‘k’.
This is a dimensionless number that is the “phase angle” of the light. (See https://www.quora.com/If-the-wave-is-completely-in-phase-the-circumference-of-the-orbit-must-be-equal-to-an-integral-multiple-of-the-wavelength-λ )
(distance from start)/wavelength = phase angle/