What is a "stage direction"?

A note in a play which tells you what is happening visually on stage
A note in a play which tells you what direction a character is facing
Introductory materials given to an audience attending a play
A pre-play speech given to acknowledge the author

say the correct answer and explain why

A note in a play which tells you what is happening visually on stage. This is the correct answer because a stage direction is a note in a play which provides instructions to the actors and crew about what is happening visually on stage.