The article discusses different methods for fitting the yield curve, which is a way to understand interest rates over time. Traditional models don't always match real market rates, so new techniques are needed. Researchers found that market yield curves have more varied shapes than predicted by existing models. To make these models more accurate, they need to be adjusted to match actual market data. This can be done by calibrating the model to market instruments like money market products or interest-rate swaps. Alternatively, the yield curve can be created from market rates and then the model can be adjusted to fit this curve.