Exactly? I can do exactly. I don't know what your science/tech level is, so let's start with a few basics.
High school geometry, continuously curving line drawing, usually shown going up and down with respect to a line through the center. That line is important. A sine wave frequently represents the movement of something. Coming and going, ebb and flow, push and pull... The part above the center line represents something (energy, water, electrons, etc.) moving in one direction, and the part below the centerline represents the same things moving in the other direction. For a perfect sine wave, the average value is zero. That is, the total area of the part above the centerline equals the total area of the part below the center line.
There are an infinite number of other types, or shapes, of waves, and some of the more common have their own names. Square wave, triangle wave, sawtooth wave all "look" pretty much like their names. The normal state for each of these waves is just like the sine wave - the average value is zero, and the waveform is symmetrical about the centerline.
In electronic circuits, the centerline mentioned above is called the reference potential. In many circuits, it also is called "ground". Technically, the reference is "at ground" only if there is a wire connected to something buried in the earth, like a cold water pipe. However, it is common usage to refer to the reference of any circuit as "ground" even if it is not connected to the earth (a condition called "floating").
Any measurement of any thing, anywhere, at any time in this universe MUST be done with respect to something else. My house is 15 miles from where I work, but 50 feet from my next door neighbor. So measuring where my house "is" only makes sense if you know the starting point of the measurement, the reference. Does the bottle have one gallon of water, 128 ounces of water, or 7 pounds of water? Depends on what you are measuring, on what the reference is. When measuring voltage, the meter always has two leads. So you are always measuring a voltage *with respect to another voltage*. By concensus, we refer to one as the input and the other as the reference.
The movement of a piston is a car engine describes a perfect sine wave. With respect to the middle of the cylinder (the reference position), the piston spends as much time extended above the reference as it does retracted below the reference. Nice symmetrical sine wave, zero average value - with respect to that particular reference.
BUT, lets move the reference position to the center of the crank shaft. Now the piston spends all of its time "above" the reference. The stroke of the engine (the distance the piston moves from its most extended to least extended positions) is unchanged, but all distance measurements read very differently from before because the reference has changed. All measurements are OFFSET by a fixed amount, the distance from the crankshaft to the center of the cylinder. So, with respect to this new reference, the motion of the piston is an offset sinewave.
A piece of electronic test equipment called a function generator can produce voltage waveforms with the common shapes mentioned above. It also can add to the output signal an adjustable offset voltage. So a square wave which normally would oscillate back and forth between +2.5 volts and -2.5 volts could have 1 volt of "positive offset" added to it, and the output would oscillate between +3.5 volts and -1.5 volts. The peak-to-peak amplitude of the wave (like the stroke of the piston) still would be 5.0V, but the offset might change how the signal performs in a circuit.
If the offset is greater than 1/2 the the peak-to-peak value of the wave, then you have a situation where 100% of the wave is above the reference. For example, an 8Vp-p (8 volts peak to peak) square wave normally would swing between +4V and -4V. With +4.25V offset, it would swing between 0.25V and 8.25V. This is approximately the output of a typical Zapper circuit. The output is a *continuous* square wave, about 7 to 8 volts peak to peak, with enough offset so that the negative peaks never go "below" the reference.
What this means in a physical sense is that electrons always are moving from the reference terminal to the output terminal, and never in reverse direction. An analogy would be turning a faucet full on, then almost off, then full on... At no time does the flow of water stop, or reverse and go back up the pipe.