4 Pin male to female Fan Power Cable
Its detecting the speed of whatever connector is on the motherboard. Some motherboards have more than one fan connector. When the fan is connected on the motherboard with the fan splitter its simply reading the fan speed of one fan
Note that you can use one of these just as an extension cord alone, but given that it has the capability of yielding two power sources from one source, it is especially useful, and especially if you are buying it to have an extra power cable for unknown future purposes (which might require a splitter, or might simply require an extension).
If you use only one of the two output cables, you'll want to remember to tie up the unused wire very securely to keep it from getting into other stuff such as fans inside your case or coming to rest upon a very scalding hot chip that could melt the insulation of any such cable.