This step is only valid for reports that are being created from a template. When using a template report you must provide the wizard with values for certain labels that might appear in the report. Templates will also have custom headers/footers that reference certain columns from the query. Since the template cannot know what column name you might have you must provide that mapping in this step.
Here we are setting the 'Report_Title' Mapping Key to the "Product Line Report"
There are three different types of mappings. A label mapping is simply a straight text replacement used only for display purposes. An item or detail mapping is used to map a column which will appear in the items section of the report. A group mapping is used to setup report groups. When the report groups are mapped, the wizard will automatically update the report to use the desired columns as groups.