Reuters details what the index shows:
Zero values in the index indicate a national economy expanding at historical trends, while negative values indicate below-trend growth and positive values signal growth above trend, the Chicago Fed said. The 85 economic indicators that comprise the Chicago Fed's index are drawn from four categories: production and income; employment, unemployment and hours; personal consumption and housing; and sales, orders and inventories. The three-month moving average provides a more consistent picture of national economic growth compared to the more volatile monthly index.And the three month average fell to -0.64 in February from -0.04 in January (i.e. below trend growth).
Source: Chicago Fed