Training Monotony represents a statistical variation in your day to day activities.
It has long been recognised (and is fairly logical) that hard training sessions should be intermixed with easier ones or rest to allow recovery and physiological adaptations to take place. Training Monotony is one mechanism to measure this concept. This makes a statistical analysis of your activities to numerically represent the variation. Research would suggest that monotony figures below 1.5 are desirable, with figures above 2.0 as potentially being cause for concern.
Further Reading
Training Strain.