The color and arrows show the surface currents throughout Puget Sound. The strongest influence on the currents is the tides, although winds may also have a big effect, especially near river plumes.
The movie has a panel at the bottom that shows time. The tide is evident in the twice-a-day variation of the sea surface height. Daytimes are shown as the thick yellow lines on the horizontal axis. Winds are shown by an arrow in the middle of the map, with the scale given by the circle.