To answer this question it is easiest if we go back to basics with the fundamentals of light.
Light is part of the electromagnetic spectrum and this is measured in wavelengths using a nanometre (nm) or 1 billionth of a metre. The general rule is the shorter the wavelength the more energy.
Humans can see visible light which is between 400 nm and ~730 nm. We cannot see UV (ultraviolet) light which is between 290 nm and 400 nm.
The solar transmission through shade cloth is significantly reduced across all wavelengths and is dependent on colour. All colours reduce UV transmission, which is the cause of sunburn and skin cancer, by at least 90% and up to 95%. This is measured by UVR block. Heat or infrared is also significantly reduced, particularly by the dark colours which reduce the heat transmission by up to 90%. Shade cloth provides excellent protection against the harsh effects of the sun.