Density means how closely spaced your workouts are.
High density indicates that you’re executing high-dose (hard) workouts close together. e.g. intervals on back to back days or only spaced by one day.
Low-density training spaces out high dose workouts with several low-dose days. e.g. 3 easy days between high-dose workouts.
Examine the image to the left. The top week and the bottom weeks have identical workouts, but their density is dramatically different.
The top week has a greater density than the bottom week. In the top week, Tuesday and Wednesday have hard workouts in the morning with afternoon endurance rides culminating in another hard day on Thursday. That’s 5 workouts, 3 of them intense, in 3 days.
The bottom week has the same 5 workouts as the top week, but spaced out over 5 days.
Even though the weeks have the same workouts, the training effect and execution difficulty between the top week and the bottom week are so different that they may as well be different weeks.