Az (H227) is a common temporal adverb marking a significant moment — 'then it happened,' 'at that time.' It often introduces watershed events: then God spoke, then they sang, then Israel knew. It appears in key narrative pivots throughout the Hebrew Bible.
The word az marks divine turning points. When the Red Sea parted, az introduces Israel's song (Exodus 15:1). When Joshua crosses the Jordan, az signals fulfillment of promise. Theologically it frames history as a sequence of divine acts — God breaks into time and everything changes.