A former chemistry teacher of mine provided a great definition of “pyrophoric:”
[It] means that if you playfully squirt some at your lab mates, they will burst into flame.
In other (less amusing) words, a “pyrophoric” substance is one that ignites spontaneously on exposure to air.
Pyrophoric iron, however, isn’t as dangerous as that makes it sound, especially in small quantities.
Basically, the oxidation of iron is so vigorous that it can cause very finely divided iron metal to become incandescent. Amazing Rust has a great tutorial on how to prepare finely divided iron by thermolyzing iron oxalate, a yellow powder that can, in turn, be prepared by a simple reaction between two common chemicals.