The selected Part is mirrored across the Y/Z plane in direction of the X-axis. While the triangles of the new part have the same Y and Z coordinates as the original, the X coordinates are turned around in relation to the overall X dimensions of the part. Thus, the mirrored part has the same overall dimensions as the original, but the shape is turned around. It can either replace the old part or is positioned next to it.
Original and mirrored part
By rotation, it is possible to get the effect of mirroring a part across other planes too. If you wish a part mirrored across the X/Z plane, you have to rotate the mirrored part by 180° around the X axis. Accordingly, if you want a part mirrored across the X/Y plane, you have to rotate the mirrored part by 180° around the Y axis.