The Fuzzy flag used as a marker for ```msgstr```-strings and it means that string needs updating or fixing (and programs will not using this strings before it). After fixing such string this flag will become unnecessary and should be removed, specialized programs (PoEdit and other) do it automatic, but if you edit po-file manually then you must do it yourself.
About detailed information: it's not Geany feature, it's [gettext](https://uk.wikipedia.org/wiki/Gettext).