Bonus words are words you find that are not part of the grid in the puzzle. You will receive one coin for each bonus word you find.
Not all words are accepted as bonus words even though they are real, valid words. Part of the game is discovering what bonus words are part of each puzzle.
Also, proper nouns such as places, people, days of the week, and generally any word that is always capitalized will not count as a bonus word.
You will only receive coins for a bonus word the first time you play the puzzle. If you go back and replay a level, you will not receive coins for the bonus words you play.