'73, A very simple explanation of phase:
The current at your wall outlets is alternating. That means that the electrons flow first one direction, then the other in regular cycles. Each cycle can be looked upon as taking one revolution of a 360-degree circle. Starting from zero, the current rises gradually to a positive peak at 90 degrees, drops back to zero at 180 deg., then reverses direction and rises to a negative peak at 270 deg., finally returning to zero at 360 deg. ready to repeat on the next cycle. Assuming that you live in North America, there are 60 such cycles per second.
If you plot the level against time on a graph you get a shape known as a sinewave, and by using a device known as an oscilloscope it is possible to view this directly on a screen. You've probably seen 'scopes displaying a sinewave in the background of countless sci-fi pics. (Easy to do and looks impressive!)
Now, by using a 'scope which allows you to view two waveforms simultaneously, you can compare them. If you plugged into two receptacles on the same circuit, you would find that the sinewaves "line up." That is to say that when one is at its positive peak so is the other, when one is crossing zero toward negative so is the other, and so on.
In this case, the sinewaves are said to be "in phase" with each other. If you were to measure the voltage between the hot wires of these two outlets, you would get zero (even though each one is energized at 120V). Any time that two sinewaves don't coincide in this way, they are said to be "out of phase."
Again, assuming North America, you have two hot wires coming into your house. If you now plugged into two receptacles on different circuits, one on each of those hot wires, you would see something completely different.
In this case, when one sinewave is at its positive peak, the other is at its negative peak. When one is crossing zero from positive to negative, the other is crossing zero from negative to positive.
These waveforms are 180 degrees out of phase (i.e. to get them to line up you would have to shift one of them along by that amount).
If you now took your voltmeter again and measured between the hot wires of these two outlets, you would see 240 volts. Each hot wire is only 120V, but because the positive peak of one coincides with the negative peak of the other, there is 240V between them. That's how you get 240V for your range or dryer.
In commercial systems using 3-phase (i.e. three separate hot wires), there is a difference of 120 degrees between any two of the three phases. The sinewaves are out of phase, but not to the ultimate extent that the positive peak of one coincides with the negative peak of another. If each hot wire in this system is 120V, then the voltage between them is approx. 208V.
That's a very simplified explanation and there's far more to the subject, but I hope it gives you a rough idea of what we're talking about -- Even if it's not of much practical use in your situation!
(Note to others: I didn't even want to get into p.f. and V/I being out of phase, so please forgive my over-simplification of the topic.)