It is the processing of engraving (metal, glass, or stone) by coating it with a protective layer, drawing on it with a needle, and then covering it with acid to attack the parts the needle has exposed, especially in order to produce prints from it. – The acid corrodes or eat away the exposed (with Needle) parts. The etching process is known as biting .
The waxy resist prevents the acid from biting the parts of the plate which have been covered. The longer the plate remains in the acid the deeper the “bites” become.