T 0 is a scale factor. You can conceptualize T 0 as the width of a time window around the clock edge during which, if a data transition occurs, the flip-flop becomes metastable. The exponential term in the equation describes the probability that a metastable condition will last for time t'. As you increase the time t' that you wait before looking at a flip-flop's output, you exponentially decrease your likelihood of seeing unresolved metastability. The time constant for the exponential term is t.
Manufacturers use various forms of the metastability equation.
For example, Ref 2 uses three constants, k1, k2, and Do, giving MTBF in the form A little algebra puts this equation into the same form as Eq 1 so that T0 is and t is k2. Because metastability formulas aren't standardized, you have to read application notes carefully to understand the manufacturer's definition of each parameter. The difference between t' and t r corresponds to a change in scale factor W relative to the T 0 parameter in Eq 1. This configuration is generally the best design, but if your design violates this assumption, you have to adjust this formula. Because of these variations in parameter definitions as well as differences in the techniques used to detect metastability, it is difficult to compare T 0 across manufacturers.