Uncensored is a tag for illustrations non censored or edited. Almost all illustrations with this tag contain R-18 related contents so be very careful when searching.
While it is important for any R-18 related illustrations to be given a mosaic or censor treatment before submission, there are users who actually ignore the rule and proceeded with their submission without any edits, fully showing all private parts. In this case, the tag "Uncensored" will be necessary in order to prevent any unnecessary problems.
The submission for anything uncensored is actually allowed unofficially by the management, however for anything uncensored for 2D will be considered illegal under Japan's Penal Code 175 "Illegal under the distribution of things obscene and pornographic" category, hence almost every R-18 illustration submitted in this site is censored.
It is due to that reason, uncensored hentai illustrations are considered very hard to find, and is considered by many to be rare and valuable.
One interesting thing to note is that there are many illustrations that do not actually warrant an uncensored tag, making any genuine uncensored illustrations to be even harder to find than it already is.
While the concept of uncensored may seem stupid and idiotic to most non-Japanese people who browse pixiv, it is important to remember that "When in Rome, do what the Romans do". Hence, it is advised to submit R-18 illustrations with the mosaics or anything that censors genitalia; or R-18 illustrations with nothing censored but with an additional 無修正 tag in order to prevent things from getting ugly.