A virus is a highly infectious disease that lives off a host (that’s us). When the virus is undeterred by any medical intervention, it can grow stronger and mutate or change into a different type of virus. If it’s not interfered with at all, it will eventually grow so strong that it will kill most of its hosts (that’s us) and will have to find a new host (maybe an animal) because there won’t be any humans left to live off.
So, something has to be done to interrupt the lifecycle and growth of a virus. That’s what a vaccine is.
Basically, a vaccine stimulates the body to produce fighter cells that ward off the invading disease. If a vaccinated person come into contact with the disease, his or her immune system weakens and kills it. This means that the person doesn’t pass the disease on to their family and friends. It dies where the vaccine is administered.
Eventually, if enough people are vaccinated, the disease itself becomes manageable because it can’t be transmitted from host to host and grow stronger.
The vaccines are there to help stimulate your immune system so that you become an active part of barring COVID-19 from growing stronger and killing more people.