Explanation
In the example, cell C6 contains this formula:
=ROUND(B6,-3)
The value in B6 is 1,234,567 and the result is 1,235,000.
With the ROUND function, negative numbers for the second argument round to the left of the decimal and positive numbers round to the right of the decimal.
In this case, by supplying -3, we are telling ROUND to round the number to the 3rd place on the left – the 1000's place.