Supplementary means additional or extra, providing something that completes or enhances what's already there. When you take supplementary vitamins, you're adding nutrients beyond what's in your regular meals. When a teacher assigns supplementary reading, those are extra books or articles that add to what you're learning in class.

Think of supplementary materials in math class: worksheets that give you more practice problems beyond your textbook, or online videos that explain concepts in different ways. They're not required, but they supplement your learning by filling in gaps or giving you additional perspectives.

In geometry, supplementary angles are two angles that add up to 180 degrees, working together to form a straight line. This mathematical meaning captures the essence of the word: things that work together to complete something.

You might hear about supplementary income (money earned beyond your main salary) or supplementary evidence in a court case (additional proof that supports the main argument). In each case, supplementary describes something that strengthens, completes, or adds value to what already exists.