I would have thought the music video was self-explanatory.
Anyway, here's a quick translation of the first stanza and the chorus:
The Secret That Cannot Be Told
by Weng Li-you
"It's not wrong to love someone"
To fall in love with someone I shouldn't have
There's me and there's you
For you, I'm willing
to bear every suffering, every sweetness
To have met you on my life's journey
it was a folly fated to be
Sooner or later, this secret
will die with me
Chorus:
A secret that can never be told
Because I love you, the deeper and deeper I hide it
I hold you ever more tightly
because I'm scared of losing you
A secret that can never be told
Because I love you, the deeper and deeper I hide it
I want to let you go
and let you fly to the horizon