"So on that day Achish gave him Ziklag, and it has belonged to the kings of Judah ever since."
Ziklag was an ancient city, once part of the land gifted to the tribe of Judah when the land had been divided. The tribe of Simeon later came into possession of the city, yet the land was later captured by the Philistines before the time of King David. No one knows where the exact site of the city, however, it was believed to have been near the Philistine city-state of Gath.
While the city was originally given to the tribe of Judah, it was later given specifically to the kings of Judah. Achish was the king of Gath and in possession of this city. Upon David’s asking if he had found favor with the king, Achish gifted the city to David, who would one day become the king of Judah. This was not a temporary gift but a permanent one, hence "it has belonged to the kings of Judah ever since".
