What is resource leveling and how does it affect the schedule?

Resource leveling adjusts the project schedule to resolve resource over-allocation by delaying activities until resources are available. When the same person or equipment is assigned to multiple activities at the same time, resource leveling shifts one activity to a time when the resource is free. This often extends the project duration because activities that could run in parallel must now run sequentially. Resource leveling may change the critical path. It is necessary when resources are limited and cannot be increased.