<p>It means the user account, which upvoted your answer, was deleted.</p>

<p>Please see the Meta Stackoverflow FAQ-tag-FAQ question: <a href="http://meta.stackexchange.com/questions/126470/what-does-user-was-removed-mean-and-why-did-my-reputation-change-because-of-it">What does “user was removed” mean and why did my reputation change because of it?</a></p>