What's the meaning of GND symbol in AC circuits?
The GND (Ground) concept is quite confusing for beginner to understand AC current.
My suggestion is, try imagine the GND (Ground) actually is not ground as we're standing at, but its more like a water surface, say like sea level.
Imagine we have a special balloon, that in order to lift it from water level, to certain height, you need a X effort, same again when you try to submerge the balloon under the water down to same depth as the previous height, still you need identical X effort too, as this illustration the balloon tries to resist when submerged.
Now, imagine again, there is a process that lift the balloon up to certain N height, and then move downward and submerge it under water at the same N depth, and pull it up again and move upward, and repeat.
The word "difference" as at your DC understanding now being expanded, that its now not static as DC as it stays constant, but its a "dynamic" difference.
Yes, to measure the difference as in volt at AC, assuming the movement is constant and with constant height and depth, you need to measure at least one cycle e.g: start from water surface, upward + downward movement back to water surface, theoretically.
And yes, its getting more complex than measuring DC like just lifting the balloon and stay static at certain height.
To measure a DC is like taking a "still" photograph, but to measure an AC, still image is not valid, but must shoot in a video form, hope you get this.
My 2 cents.