Splines are curved sketches defined by control points, you can use them to create organic shapes.
Drawing a spline
1. Start drawing a line with the Apple Pencil
2. Press with the pencil to place a control point
3. Release the pressure on the pencil (without lifting the Pencil from the screen)
4. Keep drawing, and adding control points
5. Release the pencil from the screen. Your spline is ready
6. Move the control points to adjust the spline
With every press, you will create another control point (aka control vertical / CV). You can modify the position of these control points later.
If the starting point and the endpoint of the spline are connected, you'll create a closed spline.
Currently, there's no way to add or remove control points or extend splines.