It is about immunity, just not about the individual. It means that if the virus spreads to a non-immune individual from the outside the spread inside the herd will eventually stop (but we don't know when). It can still reach an arbitrary number of non-immune individuals, but given infinite time and an infinite herd, it will eventually stop.

Low level zero friction school physics analogy: think of a ball rolling onto an inclined plane. If the plane is angled upwards the ball will slow down and eventually stop rolling further upwards, if the plane is angled downwards it will just continue accelerating forever. But in the upwards case no place on the plane is truly safe from getting reached by the ball, because speed and angle is not specified. Herd immunity means that the slope is upwards, but the velocity would still be an open-ended random distribution.

A more accurate model for virus spread would replace momentum with dice rolls and slope with success thresholds, but that's just more difficult to imagine (unless your biography contains a phase of obsessing over p&p rpg rules).

With more individuals having immunity, it's likely the disease won't reach as many individuals who are vulnerable because they lack immunity and have underlying medical conditions. It's the level when outbreaks can stay localized instead of spreading to everyone.

A good friend of mine has done a lot of research on infectious diseases.

An individual has immunity when the immune response is triggered. By definition. It was very confusing to me at first before I caught on to the meaning of the term.

It makes sense when you think about it. Can you "get" the virus a second time? Of course of can. That doesn't contradict the fact that you have some level of "immunity".

After all, the immune response is a big part of what most of us feel when getting sick.

Yes, herd immunity does not prevent a given individual from becoming infected, it's a group measurement. Of course, as the rate of spread diminishes, the likelihood of a given individual becoming infected does go down.

You probably shouldn't rely on some random HN poster for your definition of what herd immunity means. It's a well defined term in the epidemiological field, and definitions are readily available via sites such as Wikipedia and search engines.