Anon, I will attempt to explain it to you.
I do not know whether you are male or female but lets assume you are male and that I know you well.
If someone asked me to describe you to them and I said "He's a man." All that says about you is that you are male, and that you are not a boy. Right?
Now, if I said, "He's a star." That says something about you. The other person immediately thinks there is something very special about you. Just by saying that short sentence I am getting the message across that you are not an ordinary person, but you are NOT LITERALLY A STAR IN THE SKY, and the other person KNOWS that. It is a METAPHOR.
Likewise, if somebody asked me to describe you and I said, "He's a millstone round my neck." The other person immediately gets the idea but KNOWS you are not LITERALLY a millstone. It is a metaphor.
I am using the words, star or millstone to illustrate the kind of person you are but the terms are just descriptive, they are not meant to be believed.
Now look at the words of the song in my previous message and see if you can spot the metaphors.