XOXO is Internet slang from the computer era typically used to express affection or good friendship at the end of a written letter, text or email. It means 'hugs and kisses'.
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Xoxo means kisses and hugs. The x's represent the way lips purse during a kiss and the o's represent arms encircling someone in a hug. This is an informal way of showing affection, usually between friends in informal letters or texting.