Answer: the relationship between yield on a bond and the time to maturity on the bond.
Step-by-step explanation:
The Yield curve is a very important tool in fixed income trading that relates the yields on bonds with similar credit quality to their years left to maturity on the bond.
This yield is very important as it can attempt to predict what future interest rates will look like due to the bonds maturing later showing different interest rates that are assumed to incorporate future risks. It is even used as a benchmark by banks to lend out money.