What does 不 mean?
不 is a Japanese prefix meaning “un(~), non(~), negative prefix” in English. It is written as 不 in kanji, ふ in hiragana, and fu in romaji. This word is part of JLPT N3 vocabulary.
Example sentence
１日でローマを見物することは不可能だ。
It's impossible to see Rome in a day.