A mirror curve is a mirror image of the original curve.
Curve used to create a mirror curve
The NURBS object must contain at least one curve.
A mirror curve is created. A gizmo (yellow by default) indicates the direction of mirroring. Transforming the mirror curve's gizmo changes the orientation of the mirror, letting you mirror along an axis that isn't aligned with a local coordinate axis.
In viewports a gizmo (yellow by default) indicates the mirror axis.
The Mirror Axis buttons control the direction in which the original curve is mirrored.
You can't transform the mirror curve directly (that would simply transform the mirror curve and its parent curve at the same time). You transform it by transforming its gizmo. By using transforms you can mirror about an arbitrary axis, rather than using one of the Mirror Axis presets. When you transform a mirror curve, you are actually transforming the mirror plane, so Rotate has the effect of rotating the plane about which the curve is mirrored. (This is like rotating the mirror gizmo in the Mirror modifier.)
The Mirror Axis buttons control the direction in which the original curve is mirrored.
You can't transform the mirror curve directly (that would simply transform the mirror curve and its parent curve at the same time). You transform it by transforming its gizmo. By using transforms you can mirror about an arbitrary axis, rather than using one of the Mirror Axis presets. When you transform a mirror curve, you are actually transforming the mirror plane, so Rotate has the effect of rotating the plane about which the curve is mirrored. (This is like rotating the mirror gizmo in the Mirror modifier.)