Examples (rounding all of these to the one's place):
2.51 becomes 3 (this is the 'nearest' rule, which always comes first)
2.49 becomes 2 (again, 'nearest')
However, what happens if you have 2.50 ? Which way do you round it... 'tis no nearer to 2 than to 3. The IEEE standard says if there is a tie, round to the even number.
2.50 becomes 2
3.50 becomes 4
You have to pick up or down... this method is consistent and thus tends to make your errors (statistically) smaller.