This is a better relative measure of the fragmentation of your sleep over time compared to total awakenings. For example, 4 awakenings over 8 hours of sleep (0.5/hr) is very different than 4 awakenings over 4 hours of sleep (1/hr).
Rules of thumb
Less than 0.37 is one indicator of good sleep quality
More than 0.68 may indicate poor sleep quality