When the text colour is close to the background, a light text-shadow offset 1px down (and a dark one 1px up) makes the letter's bottom edge look lit and its top edge look shadowed — reading as pressed into the surface rather than sitting on it.
It's the visual opposite of neumorphism's "raised" look, and it only works subtly — 1px offsets, low opacity. Push it further and it just reads as a blurry double shadow.
When to use
Use for headers on a same-tone background, skeuomorphic UI revivals, card labels. In dark mode the light/dark order needs flipping to look right.