OLAP cubes are descriptive representations of multidimensional structures. They consist of key figures(facts) and dimensions with attributes assigned to these key figures. This allows the user to view the key figures from very different angles. For example, he can switch from the view of the key figures over time, to the view of the key figures by product groups or by sales regions. This process is called rotation, because you figuratively switch from the view of one dimension to another, like turning a cube.
Tools for viewing OLAP cubes in pivot tables have functions to rotate the view by swapping the dimensions in the axes of the pivot table or (more visually) by swapping the horizontal and vertical dimension axes.
The rotation is needed for the analysis technique of slice and dice.