the axis are the coordinates X,Y,Z, if you look closer to a view, ex, persp, then on the lower left corner you will see a: X,Y,Z color thingie, if you move an object up, it is moving in they Y direction etc, mirror +X will mirror towards the X handle of the axis, mirror +Y will mirror upwards of the Y handle in the axis,, it is kinda self explaining

sorry for confusing you