Can someone explain to me what it means to say an ac wave is "riding on a dc level"?

Question

I was reading this electricity book and the authors stated that signals can be ac, dc, or ac riding on a dc level. What does this third waveform technology mean mean?

Answer 1

Let's say you have a 0 volt dc with a 20 volt p-p ac riding on it. You have a plus and minus 10 volts.

Next take a +5 volt DC and apply a 20 volt ac. You have plus 15 and minus 5 volts.

Then take a 20 volt DC an put 10 volts on it. Now you have plus 30 and plus 10. the current no longer reverses direction.

look at the examples going through a diode.