If someone tells you not to give up the day job, they are saying that they think you should continue doing what you are good at, rather than trying something new which they think you will fail at. One's primary job, usually at daytime and enabling one to pursue a secondary activity from which one would eventually like to draw an income: "She took a day job as a bank teller in Lubbock; nights, she played in pickup games, but she didn't have a trainer, a coach, or a team" (Elizabeth Royte). your normal job which you earn most of your money from doing, especially as opposed to another interest
Used to criticise a person's talent (especially musical, singing or acting,) implying that they could not earn a living from it without other regular employment