Fresh Off the Boat
It refers to (mostly) Asians who just immigrated to the US and speak with a heavy accent and have a lack of understanding of the American culture. Of course, this would refer more to the older generation, as many teenagers and young adults now would hardly find themselves too alienated if they came here. Many elements of Asian pop culture draw off of American influences, so they're pretty in-the-know nowadays.
FOB was originally used as a derogatory term for European immigrants around the 17th century or so. More recently, it's used mostly on the West Coast to refer to Asian immigrants.
However, the term can apply to any immigrant who lacks an understanding of the culture and traditions of the nation they have immigrated to, as well as lacking a firm grasp of the country's primary language.
It's starting to be used in fashion more recently, though, where the "fobby" look can refer to trends started by Asian pop idols, mainly including hairstyles (Gatsby moving rubber, anyone?) and outfits that wouldn't be seen as commonplace in the US, but would still classified as trendy by most people. It's a look that's overall cutesy/pretty, yet maintains stylishness, as many Asian nations (I'm mainly referring to countries like Japan, Korea, etc.; I don't know too much about the Middle East) have an almost obsessive fixation on pretty (yes, applying to guys, too) looks. It hasn't reached mainstream popularity in the US mainly since it's not a look that many non-Oriental people could pull off properly with ease and not appear a bit ridiculous (I've seen it done, though).
It's an acronym for several other things as well, as the other answers have addressed, but when you'd use FOB derogatorily, It most commonly refers to fresh of the boat.