A flat curve represents confusion on the economic horizon - the markets don't know what to make of the interest rate environment, and this is usually accompanied by a choppy equities market. Finally, there is the most recent formation, aka the inverted curve. This occurs when short term rates are pushed higher than long term rates and almost always points to a future recession. Because of this an inverted curve is ominous for equities. As the Federal Reserve pushes short term rates higher this increases the short term borrowing costs for corporations and individuals.
Individuals spend less and corporations lose more on the bottom line because of the higher rates.
To show a better picture I created a 3-D chart which shows the yield curve progression from September 1999 through today. On the first chart below it goes from September 1999 back to the current date. Here the time axis moves from the current chart date back to September 1999. The reason I created both charts was to give a full perspective of the yield curve activity through this time period. My apologies for this.