I just wonder what's the '0' at the end stands for?

From Excel's built-in "Help" for the ROUND function:

ROUND(number, num_digits)
number: The number that you want to round.
num_digits: The number of digits to which you want to round
the number argument.

So num_digits=0 means "return a value having no digits after the decimal point." In our case, that's complete days with no extra hours or minutes. If we were dealing with dollars and cents, we might use 2 instead of 0.