In general, it means that you worked hard as an intern or during the first year of your residency. As each year passes, you get worked less.
For example, neuro program x has their first year neuro residents take call every four days for 11 straight months. They also don't have any in-house backup. However, during their second year, they're q7. During their last year, they are q15.
Program y on the other hand has their first year residents take call every 7 days which we all agree is much more humane. However, these residents take q7 call for all 3 years of their residency.
That's my understanding of "front-loaded."