What do they mean in flutter by 'cross axis'?

For a column the main axis is vertical and the cross axis is horizontal (90° to the main axis).
For a row the main axis is horizontal and the cross axis is vertical (90° to the main axis).

MainAxis is the axis for the Widget in which it is supposed to scroll.

CrossAxis is the one which is perpendicular to MainAxis.

---

For Row:

mainAxisAlignment = Horizontal Axis
crossAxisAlignment = Vertical Axis

---

For Column:

mainAxisAlignment = Vertical Axis
crossAxisAlignment = Horizontal Axis

Image source