Iron and copper metals are coated by tin plating for their protection form corrosion and poisoning, what is the meaning of poising here? Thanks a lot.
When iron is exposed to the air - especially in the presence of ample moisture - it will undergo an oxidation reaction, resulting in rusting. The rust is essentially Iron(III) oxide, and excessive amounts of this entering your body can be detrimental to your health.
Same thing with copper -- while it doesn't "rust" per se (it corrodes, but doesn't exactly rust), excessive copper entering your system can be hazardous to health.
The tin coating not only keeps you safe, but also ensures that the metal lasts longer, which saves money, infrastructure, and so much more.
There's plenty more articles that talk about this that you can look up, if you're interested.
Hope that helps :)