The yield curve plots interest rates for securities of the same credit quality but with differing maturities. The most widely referenced yield curves is the U.S. Treasury curve, which plots the yield for treasury bonds over a range of maturities. The current yield curve can be viewed here and compared with yield curves on different days.
A flattening yield curve is typically a sign that the market is expecting weaker growth in the future, while a steep yield curve expects robust growth. An invested yield curve often precedes a recession.