Question

What is meant by the ratio of the sine of the angle of incidence and sine of the angle of refraction is constant

Answer 1

it's a constant called refractive index which is a material property.
the ratio of sin(i)/sin(r) = refractive index is called Snell's law
it depends on wavelength of the light

Answer 2

the ratio of sine of angle of incidence to the sine of angle of refraction is a constant for the light of a given colour and for the given pair of media .This lawis also known as snell's law of refraction

if i is the angle of incidence and r is the angle of refraction then

sin i/sin r =constant

this constant value is called refractive index of second medium with respect to first