To serenade means to sing or play music for someone, especially as a romantic gesture, often at night under their window.
Therefore, "serenaded" means someone was sung or played music for, typically as a romantic gesture.
Here are some examples:
* "The knight serenaded his lady love with a beautiful ballad."
* "The band serenaded the newlyweds after their wedding ceremony."
* "He was serenaded by his friends on his birthday."