Modifiers transform an object in a non-destructive way: they sit on top of the mesh without altering it permanently. You can adjust or remove them at any time.

Three genuinely useful modifiers

• Subdivision Surface: automatically smooths and rounds off angular shapes.
• Mirror: mirrors your changes symmetrically. Ideal for modelling a face or a car, since you only ever work on one half.
• Array: duplicates an object in an evenly spaced series (a row of railings, a fence and so on).

You add them from the wrench tab in the Properties panel.

Key takeaway: modifiers are huge time-savers. Being non-destructive, they stay editable for as long as you do not « apply » them.