How does herd immunity protect people who

cannot be vaccinated?

The resistance that is present in a population of specific area which does not allow the spread of contagious disease. It occurs only when maximum number of people are immune to that disease through the use of vaccines, this phenomenon is called herd immunity.

Herd immunity protect people who

cannot be vaccinated because most of the people are immune to the disease and this immunity reduce the spreading of a contagious disease.

Short answer: Those who cannot be vaccinated rely on those who can through herd immunity. Herd immunity is the idea that if a certain amount of people in a community are vaccinated and cannot get or carry the disease, then it won't be able to reach those who haven't been vaccinated.