It means the target black luminance on the display tab has been lifted above zero. This is now done by default, because an 8-bit JPEG can only show 12.7 EV of dynamic range, and so if we set the look curve below 0.0152, any values below that point will all get rounded to zero, and you’ll no longer be able to discern any details down there.
The hook will be highlighted with an orange indicator, and it means you have set the contrast too steep, the latitude too wide, or the shadows/highlights balance too far on the shadow side. In these cases, filmic has trouble to produce a monotonic spline, and so you will get gradient reversals in your image. To correct this, address one of the 3 points above, or reduce the target black luminance on the display tab (if you don’t mind the black clippping that may result in your final JPEG) and these actions will ease off the constraints on the spline.