Splines add curves together to make a continuous and irregular curves. When using this tool, each click created a new area to the line, or a line segment. Each click also creates what's called a control point, or points that determine the shape of the curve. And that's the gist of a spline.

Why are splines useful?

In mathematics, a spline is a special function defined piecewise by polynomials. ... Splines are popular curves in these subfields because of the simplicity of their construction, their ease and accuracy of evaluation, and their capacity to approximate complex shapes through curve fitting and interactive curve design.

What are splines used for in statistics?

Splines are widely used for interpolation and approximation of data sampled at a discrete set of points - e.g. for time series interpolation.