stepping stone

noun2

1. •
   A stone that can be stepped on in crossing something, especially a marsh or creek.

   • Falling—her veriest stepping-stone Shall form the pedestal of a throne— […] — 1829
2. •
   Something used as a way to progress to something or somewhere else.

   • 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.