Iron poisoning refers to the tendency of iron-based fasteners to slowly destroy the materials they are fastening, as the fasteners corrode.
During the surface corrosion of some metals, such as aluminum or zinc, the oxides form a protective layer around the metal beneath the surface, preventing water penetration and delaying further corrosion.
Not so iron. The principal oxide of iron in contact with water, Fe2O3, forms cubic crystals that break away from the metal and allow water to penetrate. What's worse, rust crystals are less dense than the metal itself and so the rust comes to occupy more and more space.
This is less of a problem for the outer surfaces of an iron or steel object than it is for a nail or a screw. The surfaces can easily be protected by paint or electroplating, and what crystals do form expand into the coating or the open air, and the surface can be cleaned off and recoated.
Fasteners, on the other hand, are embedded in some material. The rust crystals force their way into the material, pushing it apart. Worse, a porous material such as concrete or wood constantly wicks water to any fasteners. This causes two different types of destruction.
The first one is the one you'd expect: more water gets to the corroded fastener and creates more rust. Over time, of course, the structural integrity of the fastener is compromised. That wouldn't be so bad, because you could fix the problem by replacing the fastener.
But the second type of destruction is more insidious. Water picks up acids from the material being fastened and carries it to the rust, dissolving it, forcing the substrate apart in a different place as it dries out and the rust recrystallizes. Little clumps of destruction will slowly grow away from those rusty screws, permanently compromising the integrity of the material being fastened. The only repair is to dig out all of the ruined wood or concrete and fill in the hole to get some of the strength back.
Iron poisoning can be combatted by using the right type of fastener during construction.
The traditional method has been to use a corrosion-resistant alloy of iron: Steel will buy you some time, but steel (even stainless steel) will eventually succumb to corrosion. Also, an alloy that is more corrosion resistant is typically harder and more brittle (and more expensive) than their weaker counterparts. Brittleness is not a desirable property of a screw.
Fasteners with no iron in them have advantages in some applications. Other metals can be more expensive, or not as strong as steel. They can also corrode, but the corrosion proceeds in a different way and at least won't destroy the material. It's a bad idea for riveting metal because dissimilar metals can corrode from electrolysis. But silicon bronze (or even plain copper) is a good compromise for a wooden boat.
Another solution is to coat the fastener itself. They can be chrome plated or galvanized with zinc. The deck outside your back door may have been put down with epoxy-coated deck screws. The rebar inside reinforced concrete has typically been coated with a variety of materials: zinc, epoxy, plastic, or even rubber.
Finally, of course, it's also possible to ignore the problem and use cheap, strong, easily corrodable plain fasteners. But don't expect what you're building to last long. Rust never sleeps.