A MacGuffin is also something that is needed to advance the plot. It can be something like the briefcase with the launch codes, the magic ring of truth needed to open the door of destiny, the hair clip so the rogue can pick the lock. It's an actual object or something that needs to be gained in order to bypass an obstacle. The quest to get the MacGuffin is what (part of) the plot revolves around.
Phlebotinum is more like something that is always there, or "just is". Star Wars has the force because of midichlorians, Spiderman can shoot webs from his wrists because of the radioactive spider bite, Unobtainium was used in Avatar to explain how they could travel to another planet. Star Trek has warp drives for faster than light travel. It's the thing that explains why otherwise impossible things are possible.
Or Superman's powers are Phlebotinum, so is kryptonite, but kryptonite could be used as a MacGuffin too, like if it was needed to get a blood sample from Superman to save his life. The quest to get it makes it a MacGuffin in that case.