This is the 4563rd most frequent Spanish word.
asusta
"Frightens" or "scares."
In this sentence, 'asusta' is used as a verb (in its third-person singular present tense conjugation) representing the act of frightening or causing fear in someone or something, here referring to the effect the noise has on the dog.
El ruido fuerte asusta al perro.
The loud noise scares the dog.
Here, 'asusta' again acts as a verb indicating that darkness causes fear or unease for children.
A veces, la oscuridad asusta a los niños.
Sometimes, darkness frightens children.
In this usage, 'asusta' is employed to describe the action performed by the subject in actively causing fright to others using jokes.