This criteria refers to identifying any evidence that may prevent your conclusion from being completely true or applicable. To say it more simply, it pretty much means identifying any errors that may have affected your results and hence your conclusion. There may have been several errors that may have occurred during your experiment, such as not reading measurements properly, contaminated substances, faulty equipment etc. You'll need to say what these errors are and describe how they may have affected your results, and hence your conclusion. Then you may need to say what else needs to be done to make a better conclusion.
An example - for a calorimetry experiment where you find the energy content of something, you may have concluded that Brand A of biscuits has more energy than Brand B. However, you may have lost energy to surroundings, and hence your calculated values of the energy content may be lower than the true values. So it may be possible that the values are incorrect and that Brand B has more energy than Brand A. Further evidence may be required, by performing more experiments and averaging results to even out any discrepancies.