The Money Bin is a large building that houses a large sum of Scrooge McDuck's fortune, It also serves as an office building for Scrooge.
A 60's Disney short, Scrooge McDuck and Money, points out that it's financially unsound to keep his millions isolated. It's explained then that it only contains a portion of his money, and the rest continues to circulate. In the 1987 DuckTales, however, Scrooge seems to keep all of his money there, as evident by him being 'broke' whenever he loses the money inside. In the 2017 series, it is implied again that he keeps some of his money in stocks and businesses