The boundary edge check just tells you if your mesh is a closed volume or not. If it's not a closed volume, it just means that it can't participate in bi-directional collisions. (our collision model is currently a point-in-volume test) When working with anatomy, most of the time you'll want your tissue and bone objects to be closed volumes which is why that check exists.
The small triangle angle check is checking your triangle quality. The highest quality triangle is an equilateral triangle. Imagine you have a very long, thin triangle in your mesh, and you have a sliding attachment that is supposed to slide over that triangle. It can become computationally difficult to know what to do in such situations. It's a limitation of numerical precision.
If you run:
select -r `zMeshCheck`;
the command will select the vertices of the problematic triangles.
You should be able to frame the selection and relax the vertices a bit to make the mesh pass that check.
You can also pass zMeshCheck flags to just test one thing at a time.
select -r `zMeshCheck -smallTriangleAngles`;