What does "You're pre-approved to apply" mean?

Citibank sent me a letter telling me I was "pre-approved to apply" for a credit card. What does this mean? I thought anyone could apply, and that you only needed approval to get the card after you applied. Since I am not yet approved to receive the card, and am only approved to apply, how am I any farther along in the process than someone who was not pre-approved?
 
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