“Mechanical” keyboards use buckle-springs to facilitate the travel and, as a result, the “feel” of the keys; e.g. the resistance and the pressure profile throughout the keystroke and return.
Conventional keyboards typically rely on rubber or plastic in one fashion or another instead of the metal buckle-springs.
If you’ve every typed on an IBM Model M or similar keyboards from the 70s or 80s, those were “mechanical” before there was a distinction.