Your Samsung Galaxy S3 has a LED light indicator locating at the top left corner of the phone. You will see the light occassionally flashes with different color. Here's is a description of what each of the color mean.
When you have an event that is due, your phone Galaxy S3 notifies you with the led light indicator. You could change what type of event can send you notifcation, read this article Samsung Galaxy S3: How to Change LED Light Event Notification.