Odd wording I would say. I interpret "lose vertices" as non-manifold (corrupt) geometry: like vertices that are connected to an edge but not to a polygon.
I'm not making a video because it's shown in a course, but I can show it from the screenshot below. When I say loose vertices, it says the places I specified.
total guess but sometimes if you do a mirror operation or delete some edges there may be vertices left behind that are not doing anything anymore. In maya you find them easy by doing a subdivision preview or using cleanup tool.
It's a blender thing. They refer to it as loose geometry, and even have a special selection tool to automatically select anything that isn't properly connected. There's more info on it here where someone asked a very similar question on stack exchange:…