This quote comes from Robert Fulghum's book entitled It Was on Fire When I Lay Down on It. In the section of the book before this quote, Fulghum talks about having to answer the "what do you do" question--he calls it "your basic strangers-on-a-plane question." Fulghum is a rather non-traditional guy, so when people ask him that question or he is asked to write his occupation on a form somewhere, he often makes things up, such as being a prince or a janitor.
His point is that when he asks someone what they do, they will often just give him a business card; but what people do is not just what job they happen to be working or what career they have chosen. What we do is not who or what we are.
That is why, he says, that if anyone asked him his occupation today, he would say he was a singer. Though he is no good at it, though no one pays him to do it, Fulghum loves to sing. That is what he does. Though God did not give him the skills to match his love for singing, he still considers himself a singer. He says,
I liked being a parent to my children when they were young and had no musical standards and would uncritically sing with me. It didn’t matter that we didn’t always know all the words or have the tune just right – we made it up. We singers are not thrown by technicalities. Singers are those who sing. Period.
Of course, his larger point is that we are all many things, and very few of them have to do with what we do at work or how we make our livings. One who loves to sing is, indeed, a singer, whether he is any good at it, whether he makes any money at it, or whether anyone else ever hears him. He is a singer because he loves to sing.
This quote to me means that singing is not something that must be done a certain way. Singers are the ones who sing with their soul, not on instructions. They simply just sing because that's what singing is all about, expressing yourself.