In the example, cell C6 contains this formula:
The value in B6 is 17 and the result is 15 since 15 is the nearest multiple of 5 to 17.
As you'd expect, you can use MROUND to round to other multiples as well:
=MROUND(number,10) // nearest multiple of 10 =MROUND(number,50) // nearest multiple of 50 =MROUND(number,.05) // nearest 5 cents
And so on.