What exactly does ground mean?
A better analogy is that the circuit is the line at the edge of the clock face.

Break the circuit at 9 o'clock, and connect a battery in series. Break the circuit at 3 o'clock, and connect a lamp in series. Current flows through the circuit.

There is a voltage between 12 and 6 o'clock. That's a differential voltage, the voltage between them. It represents the battery voltage, and the voltage on the bulb.

If we call 6 o'clock 0v, or ground, as we're allowed to pick any arbitrary point on a circuit and call it any voltage we like, then we can refer to our 12 o'clock voltage as a voltage with respect to ground. It's still the same differential voltage, but we've stated what our reference is.

Let's say we have two isolated circuits. The voltage between them is undefined. If we connect a wire between one circuit and the other, no current will flow. If we now pick a point on each circuit, and connect each point to ground, that establishes a common reference, and now a voltage can be defined between points on one and the other circuit. Another wire between the two circuits would allow a current to flow.