What does the term 2 second rule refer?

It's the 2 second rule (not ruler). You make use of this rule by timing whether you are leaving enough clear road behind the vehicle in front. You find some mark ( a post, a lamp, a telegraph pole) and start counting from 0 seconds to 2 seconds just as your car is level with the mark . One good way is to say, "One potato, two potato" and notice how far you have travelled since you passed the mark. That shows the 2 second distance you travelled,

If you are behind another vehicle, start counting when the vehicle passes a post or a sign or a paint mark on the road. If you cannot count all the way to two before you are level with the same mark you are too close to that vehicle for safety.

The rule is a good one because it works at whatever speed you are travelling.