Control points are the points on the stroke that you can manipulate to change its shape.
To activate control points click on the selection mode, select the stroke you want to edit and click the Control points button on the left at the bottom of the screen.
Move control point
Once the control points are activated you can change the shape of your stroke as you pull the control points into different positions.
Increase/decrease the amount of control points
These are located at the bottom of the screen and will increase or decrease the overall amount of control points on that stroke.
Add a single control point
You can do this by hard pressing with your pencil in between control points.
Delete a single control point
You can do this by selecting the control point and then choosing the trash can icon on the right.
Select a control point and then use the brush size slider to increase or decrease the size of that part of the stroke.
This is the option next to the control points button and allows you to move control points through the different axis, rather than keeping the control points on the same plane as the one you drew on.