Retractable means able to be pulled back in or folded away. A retractable pen has a tip that slides back into the pen body when you click the button, keeping it from making marks in your pocket or backpack. Retractable landing gear on an airplane folds up into the plane after takeoff to reduce drag and help the plane fly faster.

Many useful objects are designed to be retractable so they take up less space or stay protected when not in use. A tape measure is retractable, coiling back into its case with a spring. Some dogs wear retractable leashes that can extend 15 or 20 feet but wind back up with the push of a button. Stadium roofs can be retractable, opening in good weather and closing when it rains.

The opposite of retractable would be fixed or permanent, unable to move or fold away. A retractable design adds flexibility: you get the tool or feature when you need it, and it disappears when you don't.