How to understand the return values of scipy.interpolate.splrep
This documentation may work:

BSpline

Univariate spline in the B-spline basis.

$S(x) = \sum_{j=0}^{n-1} c_jB_{j,k;t}(x)$

where $B_{j,k;t}$ are B-spline basis functions of degree k and knots t.