A non-manifold geometry is a mesh that cannot be unfolded into a flat surface with one directed normals and can not exist in the real world.
On the image, you can see the most common types of non-manifold geometry. Let’s take a closer look at each of them.
It is one of the common types of non-manifold geometry when more than two faces connected to a single edge. Usually, it appears after welding points in auto mode.
To correct you should delete excess polygon.
In this type, one or more faces are not closed and missed.
Such a geometry can not exist in the real world because it has no volume.
To fix this issue you can close the holes or add the thickness.
The hidden excess faces that closed into the object called internal faces. By facing this issue just delete the excess internal faces.
It happens when the surfaces have polygons with opposite normal directions. To fix it you should select and flip opposite normals in the right direction.
The surfaces connected to one vertex
It is similar to t-type but in this case, two or more faces connected to a single vertex. It can be fixed by separating the geometry or deleting one of the parts.