Multipath I/O (MPIO) establishes multiple routes and connections to a storage array, using redundant physical paths (adapters, cables, switches). That way, when a component fails, an alternate I/O path is used. Multipathing provides redundancy of I/O paths and can improve overall system performance.
Before multipathing is installed, you see one drive for each path setup (so with two paths, you see two drives). After multipath installation, you will see a single drive for all paths of the same drive. The drive Properties dialog, available by right-clicking on the drive in the Disk Management tool, now has a multipath tab, where the load-balance policy is set.