How to understand Mesh.GetTriangles

I have read the offical document, but i am not fully clear now! Can you explain this for me?
Any answer will be appreciate, thanks a lot!

Your mesh can have multiple materials, in which case there will be multiple submeshes. Pass in an index into the mesh you are interested in (this will be zero based). You’ll get an array back which has the indices for each triangle, so the return array will be a multiple of 3 in size.