To use a source correctly, the Oracle BI Server has to know what each source contains in terms of the business model. Therefore, you need to define aggregation content for each logical table source of a fact table. The aggregation content rule defines at what level of granularity the data is stored in this fact table. For each dimension that relates to this fact logical table, define the level of granularity, making sure that every related dimension is defined.
This shud be easy to understand if you know your Star Schema design and if you are familiar with you Fact Table. This is the centralized table in STAR which has the grain information of linked Dimension Table.
Say, your Time Dim has day level information but your underlying fact has only Month related information. In this case you have to set Fact Content - Time Dim to Month and Not Day. Setting it to Day will not throw consistency error but will throw error in reports.
You need to understand the Hierarchy and Grain, this will make you Content information easy to understand...
Click F1 after opening RPD and search for topic "Defining Content of Logical Table Sources". This will give you enough information.