FDTD convergence, FEKO

Hello dear guys and dear ladies;

I am getting convergence error whil using FDTD method in FEKO. When I searched in the internet I reached very theoratical explanations and I am not numerical mathematic expert, just using FEKO. Can you explain me what is convergence in FDTD method ? Why may I be getting this error ? What can I do to prevent this in FEKO, or in general ?

Thank you for your interest.