Backlight bleed is the backlight that are hotspots of the backlight that are bleeding through. Best way to see is to put a solid black on the screen, and than you can see where there are hotspots of backlight. For most regular people it doesn't affect them, most will only be confronted by it when having dark scenes in movies, or dark moments in games, but most people don't really complain from it. But as I do video editing, it is really bad to have backlight bleeding since that means that what I see on the screen is not the same as what I will be rendering. So it could be that I tweak the color or the intensity wrong. For the moment not a big problem since I am not doing it professionally (I only have like 20 followers on youtube, so far from professional) but it still bugs me.
Here is a picture of my current backlight bleeding, which was the best of the 3 LG 34UM88C monitors I have tested: