In drag racing contexts, a Christmas tree is not actually a tree but a stack of lights that are used to indicate when a driver should start. The exact form of a Christmas tree may vary by track, but in general there are about seven lights stacked on a column.
The amber lights on the Christmas tree light up in different time intervals, depending on the type of tree being used. As with traffic lights, the green light on the tree typically indicates that the driver should start driving. The Christmas tree name is related to the association with colored lights and seasonal holiday trees.