Herd immunity is a public health concept developed by epidemiologists intended to protect an individual and the other individuals that person comes in contact with, from becoming sick.
An official definition comes from the University of Washington: "The resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group."
An easier to understand explanation: Most of us come into contact with other people every day - family members, co-workers, friends, neighbors -- even strangers. When we take steps to become immune to an illness, then we help prevent that illness from spreading. If we aren't sick, then we also aren't contagious. By protecting ourselves, we are protecting those around us, too.
The steps we might take include being vaccinated against a disease so that we won't get sick with that disease, and therefore cannot pass it on to someone else. Childhood vaccinations are a good example of herd immunity. Diseases like smallpox, polio or measles are practically unknown today because most children have been vaccinated against them. As long as nearly all children receive those vaccinations, then the risk to the rare unvaccinated child is extremely unlikely.A less classic example is when someone stops smoking, preventing the herd from being exposed to second-hand smoke and the toxins that get into clothing, drapes or carpet or furniture. This can be especially important for protecting those who have asthma or allergies. In essence, the fewer smokers there are in a herd, the more protected the herd is from smoking-related illnesses.
A report of an outbreak of measles in the country of Gibralter showcases what happens when the herd is not immune. A significant proportion of parents had resisted getting their children vaccinated for MMR (measles, mumps, rubella). A large outbreak of measles took place within a matter of three months in 1998, affecting a large number of Gibralter's citizens.