Herd immunity (also called community immunity) is a way to protect a larger community. If enough people receive the vaccination, it prevents the spread of disease.
To stop the spead, high numbers of people must be vaccinated in order to prevent the disease re-entering the population. If enough people are immune then transmission of the disease is reduced or eliminated.
A good example can be seen for diseases such as rubella or german measles. Many cases of mothers contracting Congenital Rubella or german measles around 1964 / 1965 - lots of Deaf babies were born. Now there is a vaccine for that meaning its rare this will happen.
With the Delta variant of COVID we need a high number of people to be vaccinated to achieve herd immunity.