Jamieson, Fausset and Brown on Exodus 14:19

19. the angel of God--that is, the pillar of cloud [see on Ex 13:21]. The slow and silent movement of that majestic column through the air, and occupying a position behind them must have excited the astonishment of the Israelites (Is 58:8). It was an effectual barrier between them and their pursuers, not only protecting them, but concealing their movements. Thus, the same cloud produced light (a symbol of favor) to the people of God, and darkness (a symbol of wrath) to their enemies (compare 2Co 2:16).