Inverted Triangles are turned around. This is indispensable for repairing parts with an invalid orientation. In the Status tabsheet, you can see, if you have triangles with invalid orientation. The orientation of triangles can be seen easily, as the outside is blue and the inside is red (with default settings).
The inside of triangles is red so that flipped triangles can be seen easily.
If the part is not orientable, a dialog is opened where you will be asked if Netfabb should try to make it orientable. However, as a warning, this does not always work and may destroy the mesh. So, to be sure, it is recommended that you apply the repair before executing this function, optionally without replacing the original, and start the repair module again. Not orientable surfaces can come into being for instance because of self-intersections, or after the orientation is ignored during the stitching of triangles.
In a dialog box, you are asked whether the part shall be made orientable.