Each member of an initiative has a set capacity which is equal to the amount of available work hours during an iteration (iteration length is set in the initiative's Details). For example, in a 2-week iteration and assuming a 40-hour week, that would be 80 hours if a member is only working on this initiative.

To change a member's capacity, go to the Initiative Settings Page and select Members from the left navigation. 

Resources Workload: When the Estimate Hours feature is turned on, the progress & people tile will show the estimate hours assigned to your team and its resources. The number of assigned estimated hours will turned red if it is above the team member's set Capacity for the iteration. This information allows you to see at a glance whether your resources are under or over utilized.