We all know that light has constant speed, and variable wavelength, and frequency. Suppose I have a big red glass square sheet. When white sun rays pass through it, their colour changes from white to red. I am unable to understand this effect
First think, why is that glass sheet Red? Answer: it permits only certain wavelengths to pass through, absorbing all others. Those that passed are predominantly red, as perceived to our eyes.
White light being a combination of all colours contains the wavelength range of red, which is the only part of the spectrum that the glass lets pass through.
Essentially, white light is a combination of all colours in the visible spectrum. Now, when the light passes through the red glass, The glass absorbs all the colours of shorter wavelengths/higher frequencies and allows only red light(longer wavelength) to pass through it. I highly recommend you to study Color subtraction from this link: