after importing a mesh from Gridgen Fluent reports that left-handed faces exist. I fixed this by using "repair-face-handedness". What is the meaning of lef-handed faces, in what way do they differ from normal (right-handed) faces?

Hi, It is not a big mistake. The order of vertex number in face definition will decide whether the face is right handed or left handed. This usually happens while transfering the data from application belonging to company to other. The solver importing the mesh just has to change the order of the vertex number in face definiton.

If you have left handed cells and you are continuing the simulation without correcting it solver will calculte the face normal wrongly and that will result in wrong flux calculation. This may some time leads to devergence.