Weeaboo is a synonym of "wapanese" and means wannabe Japanese. Japanese culture, and in particular anime, manga, jpop, fashion, food and martial arts has experienced a incredible wave of International popularity. Most fans of Japanese culture view it just as a hobby and have other interests. However, some people become obsessed with Japan to the point that they want to be completely immersed in Japanese culture. In common slang this person would then be referred to as a weeaboo.
Oftentimes the weeaboo ends up moving to Japan only to find it does not meet their anticipations. Since by definition they are obsessed with Japan, they have come up with certain predefined notions of Japanese life. However, these expectations seldom mesh with reality.
Japan is well known to be a difficult society to fit into. Even Japanese people often have a difficult time fitting in. Japan is a very high pressure and socially demanding country. In Japan, they say that "the nail that sticks out must be hammered down". Japanese customs and language are highly formalized and rigid and are demanding even for those that are educated from youth in Japan.
Japan is doubly difficult for foreigners, especially foreigners intent on integrating with Japanese society. The Japanese tend to equate nationality and race. Racism is rampant and openly permitted. For example, apartment ads may say something like "Japanese only". This is totally legal and common in Japan.
After a short time in Japan the weeaboo becomes often becomes disillusioned. Some return home while others remain as they have become hopelessly addicted to the cheap supply of manga and Pocky. The ones who remain seem to get more and more bitter as the years wear on and complain incessantly about Japan. This does not make their integration with Japanese people any easier as Japanese people tend to be very proud of Japan and intensely dislike when it is criticized.
The weeaboo whiner complains about the Japanese post office, the lack of sidewalks, needlessly complex government paperwork, racism (real and imagined), high prices, the Japanese right wing, ATM machines and generally anything that they feel is better in their home countries. Despite all the complaints, they are in love with many aspects of Japan, have started to become Japanese and no longer feel completely at home in their country of origin. They are stuck in some kind of love-hate obsession with Japan. This is understandable but the whining is just terrible.
Sure, there are a lot of problems in Japan and foreign permanent residents could be a great force for change. Japan needs to get better at race relations and make laws about discrimination in housing and employment. Foreign residents are the ideal group to push for such change. However, this change will come about only when foreigners get organized and focused. Whining never accomplished anything.