References for job

Okay yallllll lol. I interviewed for a new job. They want a supervisor reference from my current job. I’m not a bad employee, I’m just worried my current job will be upset when they find out I’m looking for employment elsewhere.........what all are they allowed and not allowed to tell a potential new employer? And should I be giving them a heads up that someone may contact them...? Obviously I wouldn’t leave until I actually have something else and wouldn’t leave without giving notice.