A more easier way to explain
Polygons have a "normal" = a direction perpendicular on its surface.
These normals have to point outward of the model and must have an opposite direction...
Imagine you have a ray that goes through the object from top through bottom...
The upper polygon normal must point upwards and the bottom normal downwards.
If the ray hits 2 upward going normals first, before you get to the bottom polygon, it will cause a shadow error as seen on the helicopter (leaking shadow)
Missing polygons in the model usually show up as gaps in the shadow.