The crosses mark the daily position of the subearth point on the maps (Sun). Since the Sun's rotation axis is inclined 7.25o to the ecliptic, over the course of a year the subearth point varies (in latitude) from +7.25o above to -7.25o below the Sun's equator.
The x-axis is Carrington Longitude in degrees. Carrington Rotation
Numbers are also shown. The most recent data are located at the far left of
the map. As one moves to the right of the map, the data get progressively older.