The issue is your shutter speed in combination with the fact that the YN-560 models are all manual-only flashes. See: Why is my camera limited to a shutter speed of 1/250th when the flash is up?
Essentially, your camera controls your shutter speed by changing the size of the gap between the two shutter curtains. At your camera body's maximum sync speed, the gap between the two curtains is still big enough to leave the whole sensor uncovered. If you use a shutter speed faster than your camera body's maximum sync speed (typically 1/200s or 1/250s), you'll get black bars at the top and/or bottom of the frame because the shutters can be covering part of the sensor when the flash burst goes off, and the flash burst is a lot faster than your shutter speed.
You can get around this limitation with high-speed sync. But the only flashes that do high-speed sync are typically ones that also do TTL--the YN-560s do neither. There are reasons not to get a manual-only cheapie as your first/only flash. (See: What features should one look for when selecting a flash?).
See also: Neil van Niekerk's tutorial on high-speed sync.
1/128 is the flash power ratio. Basically, when set to 1/128, it's firing at 1/128 of full power. Manual power settings are doubles of each other (1, 1/2, 1/4, 1/8, etc.) to be 1EV apart from each other, so you can balance out the flash power against your ISO, aperture, and shutter speed.