The word passage has several related meanings:

1. A section of writing or music. When your teacher asks you to read a passage from a book, she means a particular paragraph or page, not the whole story. Musicians practice difficult passages in a song until they can play them smoothly. The word helps us talk about one piece of something larger.
2. A journey, especially by ship. Settlers seeking a new life would book passage to America, meaning they paid for space on a ship crossing the ocean. The voyage itself was called their passage. The word carries a sense of moving from one place to another, often across a significant distance.
3. A narrow way through something. A secret passage in a castle might be a hidden corridor between rooms. Cave explorers squeeze through tight passages in the rock. Your nasal passages are the airways inside your nose. The Northwest Passage is the sea route through the Arctic Ocean connecting the Atlantic and Pacific, which explorers sought for centuries.

When something is happening, you might say it's “coming to pass,” using the same root idea of movement and change over time. The passage of time refers to time moving forward, like watching seasons change or growing older.