Saiku reports allow you to generate multi-dimensional OLAP style reports. The name Saiku stems from the beautiful open source user interface for Mondrian called Saiku (http://community.meteorite.bi/) that we use to display such OLAP reports. For an introduction to OLAP and Mondrian we refer to the Mondrian documentation available online at http://mondrian.pentaho.com/documentation.
Saiku reports are easily configurable once you have your Mondrian datasource (see Section 4.8.). A Mondrian datasource defines one or more cubes. A Saiku report takes a Mondrian datasource as datasource and you additionally select the cube the report should use. This is already everything you need to use OLAP with ReportServer. For an introduction to the OLAP UI we refer to the ReportServer user guide.