A supplement is something extra you add to make something else more complete or better. When you take a vitamin supplement, you're adding nutrients your regular diet might be missing. When a textbook has a supplement, it's an extra section with additional problems or information that supports the main lessons.

A math class might use supplemental worksheets to give students more practice. A newspaper might publish a special supplement on the Olympics with extra articles and photos beyond the regular paper.

Notice that supplements don't replace the main thing; they enhance it. Calcium supplements don't replace milk, they add extra calcium if you need it. Supplementary reading doesn't replace your main textbook, it gives you more information on the same topic.

As a verb, to supplement means to add these extras. A student might supplement their allowance by doing extra chores. A coach might supplement regular practice with strength training. The key idea is always the same: you're adding something beneficial to what already exists.