FreeCAD is a parametric 3D CAD software. This means it remembers all the operations you did to come up with a shape. Some of these operations can be modified later, for example if you create a line which is 1 cm apart from another, you can change the distance to 1.5 cm. FreeCAD will then redo everything that happened afterwards to give you the new result.
As we’ll see, this means you’ll have to give precise instructions (even when you don’t know, yet) that you can amend later.