The Nights

The Nights is a song by Avicii in the album So Fresh: The Hits of Autumn 2015. The central theme of the song is (fortunately) not hidden behind ambiguous language: our speaker's father wants his son to enjoy the rest of his young, care-free years to the fullest, because his own life is consumed with the desire to be go back to that time. The first piece of evidence appears on line 8.

Son, don't let it slip away.

 In this line, it, represents life, or more specifically, his son's life. He continues

When you get older
Your wild heart will live for younger days (10-11)

Again, fairly self-explanatory. The father (speaking from experience) is saying that getting older is accompanied  by an urge to go back to a time without responsibilities. He's gotten older, but his heart is still wild, and yearns for the spontaneity and excitement of his earlier years. He continues

One day you'll leave this world behind
So live a life you will remember. My father told me when I was just a child
These are the nights that never die. (13-16)

 The nights the speaker is living will never die, which implies that the nights he will experience in adulthood will quickly fade into obscurity. Make the most of the memorable time he has left. And then the song repeats, and we experience the whole thing one more time. Personally, I found the music very enjoyable, and the lyrics have an uplifting (albeit cliched) message.