When you are cited for a traffic offense and pay the fine, you are actually posting bail and forfeiting it. The court finds you guilty, keeps your bail money and a point or two shows up on your DMV record.
FTA means failure to appear. It sounds like you signed a promise to appear, but didn't go to court on the date that was indicated on your citation. Now you can't just pay the ticket; you need to go to court and personally appear in front of the judge.