The song is based on a letter Angela Aki,a 31-year-old writer, singer and composer, wrote to the 15-year-old self. However, the idea is based on a true letter which she wrote as a senior high school student to her future self.(Kind of complicated,... anyway, that's not the point.) Here's part of the english translated lyric.

Dear You,

Who's reading this letter

Where are you and what are you doing now?

For me who's 15,

There are seeds of worries that I can't tell anyone

If it's a letter addressed to my future self

Surely I am confide truly to myself

Now it seems that I'm about to be defeated and cry

For someone who's seemingly about to disappear,

Whose words should I believe in?

This one-and -only heart has been broken so many times,

In the midst of this pain, I live the present.

I LIVE THE PRESENT.

....;

I have something to tell the 15-year-old you,

The rough seas of youth may be tough,

But row your boat of dreams on,

Towards the shores of tomorrow, just

KEEP ON BELIEVING,

KEEP ON BELIEVING,.......