8.1% is the estimated gap between what the US economy was capable of producing (in terms of GDP) and what was actually being produced in the second half of 2009. The recession that occurred between 2008 and 2009 hit the reset button on wealth derived from rising home prices and relatively cheap debt. The recession occurred during the same time as a major financial crisis in the US resulting in a severe economic downturn.

8.1% really doesn’t sound like much, but it is a large number when applied at the scale of a country (like the US). For reference, an indicator of expansion of the economy is the GDP, a mea­sure­ment where the difference between bad and good can be a few 1/10ths of a percent.