Tekoa is the name of a town or village in Judah, but this message is primarily to the northern kingdom of Israel. It seems that Amos was one of a group of shepherds who had left Tekoa to work in Israel, and while he was there at work, God gave him these words (See: [[rc://en/ta/man/jit/translate-names]])
The words "in the days of" is an idiom and refers to the time when each king reigned. Alternate translation: "when Uzziah was king of Judah, and also when Jeroboam son of Joash was king of Israel" (See: [[rc://en/ta/man/jit/figs-idiom]])
The assumed knowledge is that the original hearers would be aware of when a large earthquake had affected the area. (See: [[rc://en/ta/man/jit/figs-explicit]])