XOXO is a symbol or a series of letter which means ‘hug and kisses’. In some cases, X is representative of kiss and O is representative of hugs, while in some the reverse is true. In modern letter writing such as in mail and emails (or even chat), XOXO is a sign used to convey affection or used as a sign to sign off. In many cases is it used to replace ‘love’, ‘sincerely’, ‘yours’, etc. before signing the name.
The X part of XOXO is thought to have come from the early pre-literate era where the illiterate would use X to sign documents, a sign representative of the cross. Some claim that this became a sign for a kiss, because the X would routinely be kissed since it is a holy symbol, much like how one would kiss a crucifix or the bible.
The O is thought to have come from illiterate Jews who preferred signing their names with an O to avoid signing the cross. Some however have a simpler explanation for this, such as O looks like a pair of arms going around a person in a hug.
XOXO is made popular by mainstream media today because of the TV series Gossip Girl, wherein the narrator of the story always signs off with XOXO after every episode.