Dimensions and relationships, not fixed shapes
In parametric modeling, a part isn't defined as a fixed, frozen shape, it's defined by a set of dimensions and relationships that the software keeps track of. A bracket might be defined by its length, its thickness, the diameter and spacing of its mounting holes, and a rule that says those holes stay centered no matter how the bracket's overall length changes. Change the length dimension from 40mm to 45mm, and every feature that was defined relative to that length, the hole positions, an edge fillet, a slot, updates automatically to match.
This is a genuinely different way of working than drawing a shape freehand. The relationships between features are the actual content of a parametric model, not just the final shape. Two brackets that look identical on screen can be built completely differently underneath: one where the mounting holes are locked to always sit a fixed distance from the edge regardless of length, and one where they're locked to a fixed absolute position that would leave them stranded in the wrong place if the length changed. Which relationships you choose to define is a real design decision, not a cosmetic one.