When long term interest rates fall, homeowners refinance, trading in their higher rate mortgage for a lower one. A tax advantaged fund that invests in bonds issued by state and local government units. Most have bonds from a variety of places, but some stick with issues from just one state or city providing less diversification, but an additional tax advantage. You can choose from funds specializing in short or intermediate term issues or others offering a variety of average maturities. They typically pay more because they are riskier.
Some funds add a few bonds of higher quality into the mix.
The wild card here is the value of the U.S. dollar compared with other currencies. If the dollar's value drops in relationship to a currency, and the fund holds bonds from that country, you'll be on the sunny side of the currency exchange rate. Of course, the reverse also is true.