Oops! is an exclamation that means you've just made a mistake or caused some kind of problem, e.g. messing something up, breaking an object.
It can also be used sarcastically, when you deliberately cause a problem. A specific example would be spilling a drink on somebody's work because you want them to start again. In this context, the word 'Oops!' is said with less shock (especially if it is meant to be obvious that it the deed was deliberate).