"X" is as stated above just a short abbreviation of throwing a kiss ,although it is not clear what 'x' has to do with the word 'kiss', there might be a relevance that I don't know about. 'Xx' refers to kisses. Usually 'x' is associated with 'o' ,which refers to hugs ,so you would often be sent 'xo' kisses and hugs. xoxo :) Cherry

It's not a word, it's just a way people represent a kiss when texting.

If it's at the end of a text, it can mean kisses.