The Resource Allocation view is a combination view with the Resource Usage view in the upper pane and the Leveling Gantt view in the lower pane. The reason this view is so useful is that the Usage view in the upper pane does a great job of showing exactly how many hours have been assigned to a resource during a specific time period and the Leveling Gantt view shows the task in question and also a graphical representation of how far the task can be moved out in time without delaying the project. This lets you see if you can move the entire assignment to a later time, and what effect this move would have on the project finish date.
Showing and Using the Resource Management Toolbar
Right-click any toolbar and select Resource Management.
Show the Resource Allocation view.
Move the selection in a view to the next overallocation .
Finding Overallocations
Click the Go To Next Overallocation button to have Project select the next overallocation.
Project moves to the first overallocation.
You can see that the overallocation is caused by Brian's assignment to Task 1 and Task 2.
This line shows the slack for Task 1. This means that Task 1 can be moved out one day and not affect the project finish date.
Resolving Overallocations
There are several ways to resolve overallocations, including delaying an assignment or task so that it starts later, moving the task or assignment to start sooner, decreasing the assignment Units value so that the resource is not overallocated, or replacing the resource with one that will not be overallocated.
Delaying an assignment or decreasing the assignment's Units value both risk pushing out the project's finish date because they both generally push out the finish date of the task. The effect of such a change should be watched closely. The safest methods from a schedule standpoint are either to move the task so it starts sooner (if possible) or to replace the resource. Neither of these methods have a direct impact on the project's finish date, and in some cases starting the task earlier can even hasten the project's finish ”which is always a good thing!
Right-click any toolbar and select Resource Management. 
Show the Resource Allocation view. 
Move the selection in a view to the next overallocation . 
This line shows the slack for Task 1. This means that Task 1 can be moved out one day and not affect the project finish date. 
