Statsmodels is Python's inference-focused statistics and econometrics library, sitting on top of NumPy, SciPy, pandas, and Patsy. It matters because it combines model estimation with the outputs practitioners actually need for analysis: standard errors, hypothesis tests, diagnostics, confidence intervals, and interpretable summaries. A useful mental model is that statsmodels is less about black-box prediction and more about specifying the model carefully, checking assumptions, and defending the conclusions after fitting.