What is Resource Leveling and Why Do You Need it?
A Guide to the Project Management Body of Knowledge (PMBOK Guide) defines Resource Leveling (also known as Predictive Planning or Priority-Based Scheduling) in the context of project management tools as:
A technique in which start and finish dates are adjusted based on resource constraints with the goal of balancing demand for resources with the available supply.
Many project management and project planning tools force you, the user, to specify when you think tasks will start and end. For example drawing bars on a Gantt chart. "Bob will do the web design full-time from Monday to Friday next week".
"Bob will (also) do the browser compatibility test, full-time, from Monday to Friday next week". Bob can't do two things full-time at once, so you've just created a plan which is unworkable. Plans for IT projects are never very accurate, but to create a plan which couldn't even work, even under ideal conditions, is a waste of everyone's time.
Some tools "solve" this by offering you "warnings" when people are over-booked.
You tell the tool the people's availability, and the amount of effort predicted for each task, and the tool will tell you when the piece of work is predicted to happen.
Computers are good at doing calculations.
In the example above, you say "Top priority is the web design, that's going to take a full work week. Next priority is the browser compatibility test, that's also going to take a full week". The tool will tell you the web design will happen next week, and the browser compatibility test the week after.