My friend from the UK uses it a lot in messages to his Ukrainian girlfriend. He explained me that 'XXX' means 'kisses' (an expression of his love), and 'xx' is an expression of the friendship.

As they have already said xx means kisses. You can also see "xoxo" which means kisses and hugs :)

Yes, kisses, each X means a kiss, we use them when we write to our partners, friends and relatives.

It means "kiss kiss". Lucky you! :D

Thank you! Now it's clear how to use it and not to find myself in embarrassing sitiation ;)