So as many MyDays know or probably deduced from the song lyrics and the MV, the song is most likely dedicated to Junhyeok, who left Day6 after their debut.
At first the song seems to be about losing someone, and sure it can definitely be interpreted as such. Upon watching the MV further, Day6 left a few subtle clues that the song and MV is dedicated to Junhyeok
Right at the start of the song we see a clear song of an empty mic stand
Empty keyboard, which was Junhyeok's position, now taken over by Wonpil
6 lockers
And the lyrics of the chorus, translated as such
I’d been holding on to you for so long
But now I must let go
There’s nothing I can do for you
It’s the only way to make you happy
So I let go, let go, let go
Holding on to you
Will do you no good
I know, so I struggle to get you out
The times we had together, our dear memories
I let go, let go, let go
So you can smile someday
And Brian's Young K's rap
Ah let go, let go,
The bright future we sought together
I know I know we can no longer
Wish for a happy ending
Like the land hardens after the rain
Pain is only temporary, someday you will meet
someone who can make you happier
That’s the kind of love you deserve
I got to say good bye right now
The MV is really simple, but it has a deeper meaning to it. Sometimes I wonder how things might have been like if Junhyeok stayed
While you are at it, check out the lyric video
And also the rebooted version
