I developed some Ilford FP4+ and on the edges it reads "Ilford FP4+ Safety Film". What does the safety film bit mean?
The predecessor to safety film; nitrocellulose film (nitrate film), based upon guncotton was by and large the most common base before Kodak began working with acetate film in 1948, bringing it to market in the early 1950's.
This acetate film was marketed as safety film, for it did not decompose as nitrate film did. Nitrate film was inherently flawed in that, over time, it would decay into nitric acid, which catalyzed its decomposition to an even greater extent. Furthermore, the all too common theatre tragedies of the early 20th century were in large part due to nitrate film catching on fire. If the film happened to get stuck in a carbon arc projector (common at the time), chances are it would begin to burn. For all intents and purpose, there is no putting out a nitrate film fire; it burns so furiously.
Acetate film was intended to rectify these often fatal flaws. While nitrate film was being phased out, acetate was marketed as "safety film" in order to distinguish it from the former. As I'm sure you can tell from my example though, this issue with film had largely to do with cinema, and from quite some time ago. It begs the question: Just how old is this FP4+ anyways?
It means the underlying film stock is NOT nitrocellulose (aka cellulose nitrate aka "nitrate"), which is dangerously flammable. But it seems silly now, the last nitrate film was produced in 1950.