Joe is right, I'll add a little more.
Phantom voltages come from two sources.
The first source is capacitive coupling. That's when an energized wire runs near (usually parallel) to the wire you're testing. The electrical charge on the energized wire causes electrons to move in the wire you're testing.
The second source is inductive coupling. That's when an energized wire runs near (usually parallel) to the wire your'e testing and the magnetic field from the energized wire induces a voltage in the wire you're testing.
The difference between the two is that one is caused by the electrical charge in the other wire and one is caused by the magnetic field from the other wire. The effect is the same.
In either case, the voltage you read is real. We call them "phantom" voltages because once you try to get any current from the wire being tested, the voltages go away. The tiny induced voltage is more like static electricity on the carpet than the voltage we're used to working with. You can measure thousands of volts of static electricity but there's just no current available. Once you touch your spouse and she gets a little shock, the voltage is gone.
So when you test these things with a digital volt meter, you can measure the voltage. That's because a digital meter does not draw any current. If you tested this with an analog meter, you'd measure zero volts. That's because an analog meter draws a tiny bit of current to move the needle. The wire you're testing can't provide enough current to run the meter and the voltage drops to zero. It's' like trying to start your car from a 9V smoke detector battery. If you measured the battery voltage, it would read zero since the battery sinmply couldn't keep up with the demand.
And that's what Joe's talking about when he says to use resistors. By placing a resistor between your meter terminals, you'll place a small load on the wire you're testing and will get zero volts.
