It refers to what proportion of the population needs to be vaccinated in order to completely eliminate local transmission of disease, to snuff out outbreaks as they occur.
We’re not vaccinating children, so when we’re talking about attaining a certain proportion of the population vaccinated, we need to realize that we’re not even able to vaccinate the children at this point in time and it’ll be probably a few more months before a vaccine is readily available for all ages of children.