Left picture: An infectious disease is transmitted via bacteria or viruses from one infected and sick individual to a healthy individual who gets infectedand sick as well. This now infected individual transmits the disease to another healthy individual that gets sick etc. The larger the number of healthy individuals that are infected by one single infected one (eg. 50), the higher the infection rate and consequently the higher number of infected people in a population are found every day. This number increases exponentially by the day. This process is called epidemic (if it happens worldwide, it is called pandemic).
Right picture: If a substantial number of individuals in a population are immune against the bacterium or virus, the transmission chain is interrupted, because the immune subjects cannot get infected. Thus, the disease propagation stops. This is called herd immunity. This immunity can protect the few ones that are not immune.
There are two ways to get immunity: one is to get infected(=natural way to obtain immunity). In this case the individual goes through the disease with all its bad side effects, including the risk of death. The other way is to get vaccinated, with very little very low risk side effects. Therefore, vaccination is highly recommended. The higher the vaccination rate in a population, the safer are all members of this population.
Source: Bill Levesque: UF Health researcher explains herd immunity and how it could end COVID-19 pandemic.