Just deserts means a person gets what they deserve, often in the sense of a fair punishment, though it can also mean a deserved reward. The word deserts in this phrase is an older noun related to deserve.
That is why the phrase has nothing to do with dessert, the sweet course at the end of a meal. The similar sound causes frequent confusion, but the meanings are unrelated.
- Correct: The corrupt official finally got his just deserts.
- Also possible: After years of hard work, she earned her just deserts.
- Different word: We had ice cream for dessert.
Many people assume the phrase should be just desserts because that spelling looks familiar. In careful writing, just deserts is the standard form when you mean what someone deserves.