Most people who have used the ROUND function will have used it to the right of the decimal place.
This will round cell A1 to two decimal places. So 3.1415 will become 3.14. And 2.718 will become 2.72.
But if you can also supply a negative value as the second argument of the function.
This will round cell A1 to the nearest thousand (i.e. three positions left of the decimal place). So 1,234 will be rounded to 1,000. And 599,999 will be rounded to 600,000.
The same is true for both the ROUNDDOWN and ROUNDUP functions.
It can be quite useful in certain circumstances where superfluous accuracy is not required.