Targets allow you to compare the existing data to something you are aiming for (e.g., a budget target) within your organization.
The Modeler in EPM Live allows you to evaluate the budget or resource plan data of a selected group of projects against a target, enabling you to see whether your data is under or over your targeted values. This allows you to see how the all the current projects are doing in relationship to the targets that was set forth for the fiscal year. Manager can quickly see the red areas in the heat map which represent areas that are over budget.
An example could be a "FY13 Budget Plan" target (what you are aiming for) which would be displayed in the Totals section (bottom) of the Modeler. The colors (red, green, etc.) represent the "Target" values against the total "Budgeted" values.