For example, if you have a value in cell B5 and you want to round it to the nearest 100, use the following formula:
The effect of the -2 is to round the second digit to the left of the current decimal place. If B5 contains the value 12446, then the formula returns 12400. Similarly, if B5 contains 12472, the results will be 12500.
The same technique works to round to the nearest 10 by using -1 as the second parameter, or to the nearest 1000 by using -3.