Supplemental means added to something else to make it more complete or effective. When your teacher assigns supplemental reading, she's giving you extra books or articles beyond your main textbook to deepen your understanding. When a doctor prescribes supplemental vitamins, those vitamins add to the nutrients you already get from food.

Think of it like adding chocolate chips to pancakes: the pancakes are good on their own, but the chocolate chips make them even better. Similarly, supplemental materials support and strengthen the main thing by working alongside it.

You might encounter supplemental income (extra money earned beyond a main job), supplemental exercises (additional practice problems beyond homework), or supplemental oxygen (extra oxygen given to patients who need breathing support). In each case, something supplemental works alongside the primary thing to improve the overall result.

A synonym for supplemental is additional, though supplemental specifically suggests the extra thing is meant to enhance or support what's already there, not just add more of the same.