My darling, you must be the only person on the planet who doesn't know what that means.

It's the original "emoticon."

x means kiss
o means hug

X means big kiss
O means big hug

xxx means kisses
xoxoxo means kisses and hugs

from http://answers.yahoo.com/question/index?qid=20070613124716AAilPBZ
by Max Marie, SFO