A “day job” is a person's usual work - what they do every weekday to earn money.
Don't give up your day job 做某事不在行
Jonathan: So guess what? I remembered Sue's birthday and I made her a gift, all by myself!
Anna: Wow! Jonathan. What is it?
Jonathan: It's...a homemade hat!
Anna: Nice work, but please, don't give up your day job. I'm not sure hat-making is your thing.
Telling someone "Don't give up your day job" is another way of saying "you are not very good at something".
Anna saw Jonthan's homemade hat and knew he could not earn a living as a professional hat-maker. It is said jokingly, but can be considered rude. So be careful when using this expression.