If something is stimulated, it is made more active or alert.
Stimulated is usually a good thing. It means something is more awake or working better.
Examples:
The music stimulated the baby and made her dance.
The teacher stimulated the students' interest in science.
The coffee stimulated him and made him feel more awake.
The game stimulated their brains and made them think hard.
The loud noise stimulated the cat and made it run away.
The new toy stimulated the baby's senses and made her very happy.