It refers to the theoretical dimensions that a planar transistor would have to achieve the same density. Nobody uses planar transistors these days. Instead, 3d

It refers to the theoretical dimensions that a planar transistor would have to achieve the same density. Nobody uses planar transistors these days. Instead, 3d multi channel transistors are being used. The number is supposedly referring to the gate length of the theoretical planar transistor. If your transistors can reuse the same gate to drive three different channels, that means the effective gate length would be one third but once you open this can of worms, the numbers can be chosen arbitrarily to mean whatever you want.