Spook can mean a few different things:

1. To frighten or startle someone or something suddenly. A loud noise might spook a horse, making it jump or run away. You might get spooked walking through a dark basement when something brushes against your leg. When something spooks you, your heart races and your body tenses up, ready to run or defend itself. Animals are easily spooked by unfamiliar sounds or movements because their survival depends on reacting quickly to potential danger.
2. A ghost or supernatural being. In old stories and Halloween tales, people talk about spooks haunting houses or graveyards. This is why we say something is spooky when it feels eerie or mysterious, like a foggy night or a creaking attic.
3. Informally, an old slang term for a spy, especially during the Cold War era. Intelligence agents were sometimes called spooks because they worked in secret, appearing and disappearing like ghosts. You might encounter this usage in older spy novels or historical accounts of espionage.

Note: Spook has also been used as a racist slur. Avoid using it to describe people.