Flow is pretty much what it sounds like: How smoothly your eyes travel from one thing to the next. Elements in the picture will subtly direct your eyes in a certain direction. (NOTE: I don't know if there is an official name for this.)
If you pick a spot on the picture below, you can probably see how it flows. There are two examples below of how things connect: One is a geometric shape, showing continuity, the other is a flowing line from one point to another. By combining the two, you can see how there is a continuous flow throughout the picture.
NOTE: You can even skip the frog, and go from her face to the bottom of the scepter and then up, creating a triangle.
NOTE: The flow of this image was actually inspired by the Fibonacci and Golden Spirals. Ex1, Ex2.