Interest rates are essentially an indication of how expensive money is. When interest rates are low, money is "cheap".

Outside of the FED purchasing bonds, Banks giving loans is another way to "print money". So when interest rates are low, more people and businesses take out loans, and as a consequence there is more money circulating, "sloshing around", in the economy.

There are other factors at work though, outside of interest rates, that can cause wealth to pool.

Various forces since the mid-1990s in the United States, mostly related to the tax and regulatory environment of corporate compensation packages (i.e. paying senior employees in stock, etc), have caused net wealth transfers to the upper classes. The wealth gains those classes have achieved also causes them to have excess wealth that wants to go somewhere.