IDIOMS

stepping stone

(idiomatic) Something used as a way to progress to something or somewhere else.

Example: A stone that can be stepped on in crossing something, especially a marsh or creek.
He was full of ambition, and force, and life, intending all sorts of great things, and meaning to make his position a stepping stone to all that was excellent in public life.
Esperanto can make a good stepping stone language to Latin, which may be difficult for amateur linguists.