not certain i know what herd immunity is and how it pertains with covid-19 (my ignorance)...
It’s when enough people are immune to something that it slows down the transmission because there’s no very many people to infect to spread it.
Then the people who do not have immunity from it are somewhat protected because the disease has to find one of them to infect instead of burning out on those already immune. If the sick individual only encounters immune people the disease stops there.
The chances of an uninfected person encountering it first is lessened, and if they do get it, it’s not likely to spread far or fast because it can only spread through people who are not immune.
There may be more to it, but that’s it as I understand it.