In many tutorials, set it. But I don't understand what is it?

Thank you

It's a cell for which the residual are calculated. The convergence criteria in that cells have to be satisfied, but StarCd doesn't use only these monitoring cells, which means that the convergence criteria for a mon-cell can be satisfied without the convergenced being reached. It is better to choose the mon-cell at places where the convergence is difficult to reach. But, have a look to the StarCD method and u-guide. Laure

I don't think that description is completely accurate. The monitoring cell is simply a cell where the solution (Pressure, temperature, scalars...) is printed out (monitored) at each iteration. The convergence criteria is not based on the monitoring cell.