Sounds like they might have misbehaved lately ;)
that they are thanking you for being understanding, acceptable of their ways and never giving up on them.
It means they appreciate the fact that you love them no matter what; regardless of their faults or shortcomings.
Unconditional love means loving somebody under all/any condition. For instance, if a person had HIV/AIDS, or if that person was poor, Or sickly, Or ugly, Or fat. If you still stood by that persons side through thick and thin then you have unconditional love for him/her.