To become a successful cyclist it is important to have a good level of base fitness which is basically your body ability to process oxygen and produce energy.
Aerobic decoupling is simply a numerical measurement of aerobic efficiency and fitness. The longer you can ride without experiencing a significant decoupling the more fit you are. It is important to monitor this over the base period so that you can be sure that you have reached peak base fitness before moving onto the build phase
If you are using software sure as training peaks then the figure you need to look for is Pwr:Hr and as a rough guide you are looking for a figure of less than 5% to show good aerobic fitness, the lower the better. It is important to look at a section of steady state work and be aware that poor hydration/coffee and sleep can all affect the outcome so needs to be limited.
Heart rate drifts over time during a workout as the muscles fatigue however this can be limited during a good base fitness block.