Your just deserts are what you deserve, or the appropriate outcome for you. If you're mean to other people, your just deserts might be losing your friends.
The phrase just deserts is usually negative: "He got his just deserts after stealing all that money from the company." In other words, he got exactly what he deserved. You can also use it in a more positive way as well: "I'm so happy to see you get your just deserts after all your hard work." Desert is related to the word deserve, which is why it's spelled with only one s. The Latin root is deservire, "serve well."