That’s for sRGB, which as for most colour spaces is defined relative to CIE XYZ, which is itself defined in terms of the perception of a “standard human observe

That’s for sRGB, which as for most colour spaces is defined relative to CIE XYZ, which is itself defined in terms of the perception of a “standard human observer”. In XYZ, the Y coordinate is the luminance (brightness) of the colour and that first equation is computing Y from the RGB value