It means that the computed flow field wants to have flow re-enter the domain at an outlet. When we specify the outlet boundary condition there are fields for what the backflow conditions are. If the flow is only leaving at the exit, these values do not show up in the computation. Typically we do not want to have flow coming in at the outlet, so we will, if possible, extend the domain to a place where no backflow occurs. If backflow only occurs in the first few iterations, then it really is not a problem and just means that we had a bad initial "guess".