'Glow' can mean many things. One common usage is 'Leica glow', and this means two things, only one of which is real.

The real thing. Many older Leica lenses, while of very high quality, were not fully corrected for spherical aberrations. This gave a combination of very good rendition of detail with a surrounding 'halo' or 'glow' often quite visible in photographs with fairly large dynamic range over short distances. This is often, but not always, very attractive. Other high-quality lenses also suffered from similar aberrations.

The unreal thing. More recent Leica lenses are much better corrected and tend not to suffer from these aberrations. But people who have bought them often want them to have this magic property and will insist that they have 'Leica glow' even though this is not in fact present.

I am not saying this to detract from modern Leica lens designs: they are extremely fine lenses and the 'Leica glow' is an aberration. However if you want to see it, you need older lenses (or lenses which are intentionally designed to be imperfect in this way: some Leica lenses may be, some Zeiss lenses certainly are).