Harmonics are the flute notes of the Guitar. It’s a bit like when you pitch your voice to a Falsetto. There are a few different types of harmonics worth mentioning. First of there are natural harmonics these are the natural harmonic overtones of the instrument or the overtones we can play on the open strings of the guitar both used in acoustic and metal type music.
Next there are pinch harmonics which are the squeals you typically hear in metal music and hard rock. And lastly there are Dive Bombs or whammy bar squeals aka Mattias IA Echlundh the master of the harmonics on the guitar. Also I guess all harmonics are essentially natural even if they are heard by amplifying the sound to a perceivable volume with overdrive/gain or distortion of some sorts.
Check out the video as it shows you the most common way to play natural harmonics on the guitar. The science of harmonics has to do with string ratio which in itself is a fascinating subject but there are others out there more qualified to tell you about that in detail. One video I like in particular is Vihart’s video on Sound, Pitch, Harmonics etc. It’s very informative.