So, a quick example would be: don’t quote the population of the United States down to the ones digit. Do you remember that count down to when the population passed 300 million? It was meaningless. We can’t possibly know the population that accurately, given (a) the possibility of error in the census and (b) the fact that people are dying and being born all the time. A quick google search suggests that births and deaths are in the thousands per day. If you google “population of the United States” you get 313.9 million, accurate to the nearest 100,000.