I was curious myself and Google these days gives you the most "profitable" sites first which doesn't help but it appears to be a chemical that removes iron particles from water that usually retains it, namely very hard water, which characterises London water apparently.
I can only assume that reversal film in particular needs the removal of iron particles.
It's used instead of the final developer/fix sequence when doing b&w reversal development. It makes the remaining unexposed undeveloped silver go black after the bleach/clear baths. Cant remember the details of the chemical reaction. You can find threads discussing it's use.
"Rust Out" is a solid rust-solvent compound used for removing rust stains from textiles. It contains sodium hydrosulfite and sodium metabisulfite. The former is useful in reversal development. The compound may serve as alternative source for this chemical.
I was puzzled by this offer too, as I was ignorant on this compound.
Iron Out is a rust remover that contains sodium hydrosulfite aka sodium dithionite. 1 tablespoon is used for every 200 ml of distilled water and it is one shot. The "Iron Out" step replaces flashing (re exposing), developing and fixing.
The sodium hydrosulfite reacts with the unexposed silver nitrate left in the film – after the first development bath, the bleach and the clearing bath – to form silver, sulfur dioxide and sodium nitrate. Hence revealing the latent positive image.