I hear it all the time. They're looking for scrap? Crafting scrap? Trading scrap?
Scrap banking is the process of trading scrap metal for common, low-value weapons. Typically this is at the rate of two weapons per one scrap.
If someone needs a scrap banker, they are looking for scrap and have weapons. If someone is scrap banking, they have scrap for trade.
This is effective for the person with weapons because it means not having to go craft scrap with weapons (if they even have the correct slots to be able to craft scrap), and effective for the person with scrap because often some weapons they are given will be worth more than half a scrap.