It means a kiss . . . it's not necessarily a sweeping romantic gesture, though. Many people end letters, cards e-mails and so on to friends and family with XXXs.
The origin of the X comes from Christ's death on the cross. Since this was, according to adherants to the Christian faith, the ultimate expression of love, the cross has become, over time, synonymous with a kiss.
I'm not sure if Waldo McFroog is correct. Not only because the cross dates back before the time of Christ, but also because it is known as an 'x', not a cross. Perhaps it's to do with the mystic unknown nature of a kiss, or of love?
Firefly - I agree that the cross pre-dates Christ, but until it was picked up upon by Xians, it would have been viewed as a torture device, rather than a symbol of love? (Bill Hicks had a great riff on that subject, saying 'If Christ comes back, do you think he's ever going to want to see another f***ing cross?'. But I digress.) And one can say Xmas, where the X symbolises Christ because of his death on the cross. But naturally, I'm happy to be told otherwise.