Current always flows towards a lower potential, kind of like water always flows downhill. The only important thing here is the gradient of (i.e. difference in) potential, not the absolute potential, just like for the water only the slope of the surface matters, not its elevation.
(Indeed the absolute value of the potential is arbitrary in the sense that you can add some value $C$ to all the potentials in your system and the physics wouldn't change. In practice, we usually make a convenient choice for where the potential is $0$ (e.g. at infinity or electrical ground) and then work with that, just like we usually make a convenient choice for where the height is $0$ (e.g. at the floor or at sea level) and work with that.)
Now, ac cables are usually designed such that the potential in one wire is constant (and grounded, which we choose to be $0\,\mathrm V$) and alternating in the other one (in a $230\,\mathrm V$ power grid this will alternate between $-325\,\mathrm V$ and $+325\,\mathrm V$). In our water comparison this would be like a surface fixed with a joint at one end. You can imagine moving the other side up and down – sometimes being higher, sometimes lower than the fixed side – and water on that surface flowing from from one side to the other and back accordingly.
Just as a side note: Since electrons are charged negatively, they actually flow opposite to the current direction, from $-$ to $+$.