You can see that stop means to stop by looking at what happens when the light turns read.

It's not obvious that "STOP" means to stop either, or a "walking" character means to walk if you don't speak the language or are unfamiliar with cartoons. When you learned the language, you connected the meaning to the word, because you heard it in connection to someone stopping, so now the word stop is a metaphor for that time someone stopped.

This is exactly the same way you assign meaning to any sign, including a stoplight. That's just how semiotics works, all language is metaphorical, no sign can carry meaning without a context.