A stage is a certain phase, like the toddler stage, when kids learn to walk and talk. Another kind of stage is a platform for a performance, although according to Shakespeare, “All the world's a stage.”
There are stages of life, development, and grief — and stages of a rocket launch. Another kind of stage is the one that actors, singers, and puppet masters perform on. The profession of theater acting itself is in fact sometimes referred to as "the stage," and to put on a play is to stage it. A "stage whisper" is one between actors that's loud enough for the audience to hear, and a "stage name" is an actor's professional name.