Good question! Herd immunity means enough people are immune to the virus that it can't continue to spread through the population. That can be achieved either through a large percentage of the population becoming infected and developing antibodies (and potentially dying — not ideal). Or it can be achieved through widespread vaccination causing people to develop antibodies and immunity (much safer and much better option).
So, herd immunity is the outcome, and it can be achieved through two different paths, one of which is far preferred over the other.