This is portraying your piratical or plundering urges or what you feel plundered by, or a sense of having someone else take what is important to you. Occasionally there is the indication of hidden treasure in such dreams, and this suggests something you could find that although gained in a negative way, is actually valuable at the moment.
Pirates can also be a very unified force against which you either struggle or join. It is probably a force that disregards the niceties of social rules and regulations.
Because they are anti authority, the pirates might suggest you need a more positive sense of your own worth when you measure yourself against those who live their life within the established rules and ways.