Something extraordinary goes above and beyond what is expected. This can be good or bad. Saving a child from a burning building is an extraordinary act of heroism, but a test score of 11 out of 100 is extraordinary too.
The extra- in extraordinary means "outside" the ordinary. If you had to work for 10 hours on a two-page writing assignment, that means it was an extraordinarily hard assignment. Either that or you were extraordinarily slow. We often use the word extraordinary as a superlative. If your friend bakes you a cake better than any you’ve ever tasted, you could tell her it is “extraordinary.”