An IED is a bomb that is not made in a factory for an army. Someone builds it from whatever they can get — a pipe, farm chemicals, old wires, a phone as a trigger. The letters stand for Improvised Explosive Device: improvised means made on the spot from what is at hand; explosive means it is meant to blow up; device just means a built thing. News reports and armies use IED for bombs used by fighters who are not a regular army — people fighting a government, or terrorists. It is a cold, technical word. You will see it in the news, not in friendly talk. When people say IED they mean a real bomb, not a metaphor.