The result of a technique used to prevent the viewing of sexual parts, usually vaginas, penises and dildos but occasionally nipples. Note that the anus is not usually considered a sexual part in Japanese censorship.
Common forms include mosaiacs, black boxes, mysterious white lights, inverted colors, shaded bars, and the infamous invisicock. Tentacles or phallic objects are commonly used to resemble genitals and generally don't need to be censored, though the point of intercourse with a vagina does.
Obscenity in Japan has been illegal to some degree since the 19th century Meiji Era, but the Criminal Code of 1907 relating to obscenity was reinforced (or at least not overturned as other laws restricting freedom were) when American General MacArthur established Japan's modern laws after WW2.
From the Criminal Code of Japan (translated)
Article 175. (Distribution of Obscene Objects)
A person who distributes, sells or displays in public an obscene document, drawing or other objects shall be punished by imprisonment with work for not more than 2 years, a fine of not more than 2,500,000 yen or a petty fine. The same shall apply to a person who possesses the same for the purpose of sale.
Note that the law relates to distribution within Japan; it is not illegal for Japan-made porn intended only for export to be uncensored.