According to the author of 1 Samuel, David and his friends came back to their home in Ziklag and found it destroyed by some bad people called the Amalekites, who set everything on fire. This made David very sad and worried because he cared a lot about his home and the people living there.
1 Samuel 30:1: And it came to pass, when David and his men were come to Ziklag on the third day, that the Amalekites had invaded the south, and Ziklag, and smitten Ziklag, and burned it with fire;
God commands Saul to utterly destroy Amalek, but 1 Samuel 30:1 shows Amalekites attacking Ziklag, suggesting they were not completely destroyed.
1 Samuel 15:2-3: Thus saith the LORD of hosts, I remember [that] which Amalek did to Israel, how he laid [wait] for him in the way, when he came up from Egypt.