The brute force method for computing global illumination recomputes the GI values for every single shaded point separately and independently from other points. While very slow, this method is very accurate, especially if the scene has many small details. To speed up brute force GI, you can use a faster method (the Light Cache GI engine) for approximating secondary GI bounces, while using the brute force method for the primary bounces.
The following diagram shows the way rays are traced when using the Brute Force GI. Since the method is view dependent the first rays (Black) are traced from the camera into the scene in order to determine the points for which GI is going to be calculated. Then the Primary bounces(Red) are traced into the scene - the number of rays traced depends on the Subdivs parameter. The Secondary bounce rays (Blue) are traced only when Brute Force GI is used for secondary GI engine. In that case a single ray is traced for each bounce and the number of bounces depends on the Secondary Bounces parameter.