Is this just for people who have a MIDI Keyboard? I dont understand.

No. MIDI Yoke (or Maple MIDI from Marblesound) is what we commonly refer to as a 'virtual midi cable.' It's a small piece of software that allows you to link one midi program to another. So for people who write in a notation program like Sibelius, then launch a virtual instrument program (VDL2), the virtual midi cable is what you would use to connect them together.