路过 means to pass through. If one searches for images related to this word, one finds pictures of funny or strange situations carrying this word on them.
路过 = I see what happened and I know what you mean, but it has nothing to do with me. Or it may simply means 'who cares?'
It is a way of expressing indifference, usually used when commenting a piece of news on the internet.
News: 'Selena Gomez Admits She's Crying: Is It Over Justin Bieber?'
News: 'US unemployment rate has dropped to XXX'