J-Cuts can be described as having the sound enter the scene before the scene appears visually, or it can be used to play out a scene where the audio is used after the shot has ended. They're used in TV and movies, and they're a professional element that sets you and your style apart from those who just cut scenes together. This enables you to get a nice flow in your video, and it all blends and layers to make it more interesting and give your work more power.
This video shows how to do J-Cuts. There are two ways to use them in your video. The first way is to use audio from the first cut and blend it into the next shot. The second method is to do it using sound effects, which gives you a plethora of options. The great thing is that you can choose if it's going to be for one, two, or five seconds that the audio "bleeds" in or out of the scene.
I have used them in the past, and they really added to my projects. They're one of my tools for sure. I won't go overboard so people realize what I'm doing; that distracts and detracts from my videos. I keep it keep it delicate, yet powerful.