Usually discussions on this scripture is complete with a picture of a knight in shining armor and some darts from a dart board. That does not really help you to think about what it means.
Paul was writing to Greeks in a Greek City State. They were under the Romans, but not that far from their heritage. The "full armor" was the complete heavy armor that at one time every citizen was required to own. (Citizen applying to fully enfranchised men). There was universal compulsory military service for citizens. Each citizen was required to own, maintain and practice in a complete set of heavy armor, complete with a "professional grade" shield.
To "put on the whole armor of God" means many things, but among them it means to be a full citizen in the kingdom of God and to do your complete duty.
To "stand fast" in such a context is the duty a man in a phalanx has to stand shoulder to shoulder and not break ranks, no matter what comes his way -- for the side that fails to stand fast is the side that loses.
Fiery darts doesn't apply to the things you throw at a dart board and probably doesn't refer to lawn darts either (the military equivalent). It might apply to the large darts thrown by scorpions (a type of field artillery) or to pilum (a thrown javelin that could go through as many as three men in a row), but it probably meant tubes full of greek fire -- tubes that would roast a man alive in his armor or consume a wooden shield and the man behind it -- but that the heavy shield a full citizen (vs. a skirmisher or other auxiliary) used, triple thick bullhide, would shed, if you only stood fast.
Thinking of men, standing shoulder to shoulder, facing the ancient equivalent of napalm (except it burned underwater) and safe only if they held position together and stood fast, is an interesting image. It is a call to accept citizenship in the kingdom of God and to stand fast in spite of whatever terrors one faces.
I hadn't thought of that until today's lesson, when just what a "fiery dart" was came up and I got to thinking of standing fast in the context of a Greek citizen.
Someone said I should write this down, so I am.