The dark line that appears on the map when a herd(s) is selected are the historical movements of the herd(s); and the lighter line that appears on the map when a herd(s) is selected are the planned movements of the herd(s).
To adjust the number of historical and planned movement lines: Go to the Ranch Overview (gear) in the top left corner of the map > Layer Settings > Show Monitoring > Enter the number of historical and planned movements you want to see > Return to map