Here is the wikipedia definition of "Herd immunisation"
Herd immunity (or community immunity) describes a type of immunity that occurs when the vaccination of a portion of the population (or herd) provides protection to unprotected individuals.[1] Herd immunity theory proposes that, in diseases passed from person to person, it is more difficult to maintain a chain of infection when large numbers of a population are immune. The higher the proportion of individuals who are immune, the lower the likelihood that a susceptible person will come into contact with an infectious individual. Vaccination acts as a sort of firebreak or firewall in the spread of the disease, slowing or preventing further transmission of the disease to others.[3] Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter will not contract and transmit the disease between infected and susceptible individuals.[2] Hence, a public health policy of herd immunity may be used to reduce spread of an illness and provide a level of protection to a vulnerable, unvaccinated subgroup. Since only a small fraction of the population (or herd) can be left unvaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines because of a medical condition such as an immune disorder or for organ transplant recipients.
Basically, (and this is only my take on things) if you don't get your child immunised, it is unlikely that they will catch a life threatening disease because the majority of surrounding people do vaccinate. But the more people that choose not to vaccinate, the less protection there will be. Is it really fair to be benifiting from the immunity of others because you are not willing to vaccinate your own child? As I said, that's just my opinion but it seems unfair on those who are not able to vaccinate their children for medical reasons.