Hi Marianna,
There is some useful information in this post: How to get good mesh quality？ - SPIS - SPINE Forum
You can improve the mesh quality by using the “Optimize 3D” and “Optimize 3D (Netgen)” buttons in the Gmsh editor. For example, I will do Optimize 3D (Netgen) 3-4 times, then Optimize 3D once. Normally, I define and optimize the mesh in this window, then save it with a different name, then load and select this mesh when you get to the mesh tab.
Also, you can improve the mesh by using finer resolution around objects of interest and lower resolution on the external boundary (to keep the simulation time reasonable). The resolution is set when you define an object like a sphere or a box and the units are in meters. This sets how far apart the mesh points are on the object being defined.
As I understand, a lot of simulations will still work even if the mesh quality is bad, so sometimes this warning can be ignored.