What does the warning: 'There are elements having an angle smaller than 5 degrees' mean?

It is possible that after mesh generation, the following warning message appears:

Mesh generation

These mesh elements could give inaccurate results. Therefore it is suitable to correct them, for instance by local mesh refinement.

To track these mesh elements, view flags will point to these mesh elements with angle < 5°. Flags can be activated in View parameters settings > Misc. > Calculation info > Display arrow on small mesh elements

  View parameters settings