Red line can refer to any of the three red lines on a hockey rink. The center line of the ice is a red broken or dotted line. The goal lines are also red and these are typically red solid lines.
The red center line divides a hockey rink into to 100 foot halves. The reason it is dotted is because when hockey was first televised in black and white it made it easier to distinguish the red line from the blue lines, which were painted solid. The red goal lines are thirteen feet from the end of the rink. The nets are then positioned with their fronts at the red goal lines.