When an object absorbs all frequencies in a spectrum approximately from 380 nm to 740 nm EXCEPT the ones around a wavelength of about ~ 625–740 nanometers and at a frequency of about ~ 480–405 TeraHertz, then humans say "the object IS red".
compare: green,blue,violet,yellow,orange
(*)A "red" apple does not give off "red light", and it is misleading to think of things that we see, or of light itself, as objectively colored at all. Rather, the apple simply absorbs light of various wavelengths shining on it to different degrees, in such a way that the unabsorbed light which it reflects is perceived as red. An apple is perceived to be red only because normal human color vision perceives light with different mixes of wavelengths differently—and we have language to describe that difference. [1]