An abbreviation of a line from the intro to an
anime called “your lie in April” typical said as a sign of affection and
love.
It stands for
Kimi da yo kimi nanda yo. Oshiete kureta Kurayami mo hikaru nara Hoshizora ni naru
And translates to
It was you, it was you! The one who made me realize; if we can make even the darkness shine, it will become a starry sky.