the.com/stepping stone

means A thing, place, or position used temporarily as a means of progressing toward a bigger goal.

from Literal before it was figurative: a stepping stone is simply a stone placed in a stream or muddy path so you can cross without getting your feet wet, one dry foothold at a time. The metaphor — using something as a stage toward a further purpose — was a natural leap, recorded in English by the 1500s-1600s, and it kept the original image intact: you're never meant to stay on the stone, only to use it to get to the other side.