It’s frequently true that young people do not understand what the word “professional” means. One of my many jobs as a teacher is to tell them.
If you are a singer, being a professional is something to take seriously. Each of the styles has its own criteria, but those who are successful in that style learn what the expectations are. Jazz vocalists who are high level successful singers with careers will tell you what they think they have to know in order to be successful. Broadway vocalists will give you a different set of criteria, as will gospel singers. There are some things, though, that run through all of the vocal arts.
One is a being good at what you do without having to fuss over it. You sing on pitch, you have control over what you are singing in terms of volume and phrasing, diction and expression. You can sing when you are less than 100% well, and know how to “manage” when you are not up to par but not seriously ill. You can speak the language of music with others using the same terms and words they use knowledgeably. You can communicate with an audience in a live situation no matter what kind of a venue it is. You know how to practice and rehearse to prepare for performances or gigs. You can adapt what you are doing when necessary, and are not so rigid as to have only one way to do something, no matter how much you like it or how it works.
You know how to work with other colleagues in a respectful and efficient way. You arrive on time, prepared for your job with proper equipment, music and clothing. You do not waste other’s time by doing things that are distracting in a rehearsal. You thank the people you work with when you are done.
If you do not do these things, you might still have a career, but you would have to be either very lucky or so talented that you can outsing the rest of the universe. Otherwise, the people who are also in your area are going to find out very quickly that you are not a pro, you don’t behave professionally and you are not the kind of person they would want to work with. In other words, you won’t last.
Best to learn these things while you are young and aspiring, as the hard knocks school of life will give you a failing grade if you don’t.