When we get vaccinated against a disease this prevents us from having that disease. Some diseases are very serious (polio) or even lethal (tetanus), so it is better not to have them at all. Vaccination exposes you to a weakened version or parts of the microbe causing a certain disease, so your body “learns” how to fight the microbe without you suffering the disease. In other words, vaccination exposes you to the microbes in a safe way so you are ready to fight them if they attack your body. Vaccination is very important since it helped to completely eradicate some diseases like smallpox
I just thought I’d add that for vaccination to work well it’s important that lots of people get vaccinated. This gives indirect protection (herd immunity) to those who can’t be vaccinated because of other illnesses. It means that there are fewer people around to spread the disease to them so less chance of catching it.
Well, jamie shine, I think the other scientists already covered your question.
Sometimes we have to get a 2nd or 3rd dose of the same vaccine over our lifetime and that’s because the body’s immune response from vaccination declines over time. But did you know that the flu vaccine formulation is updated every year? The flu-virus (from the Influenza family) mutates/changes so much in a short period of time that a vaccine from last year might not be efficient to protect you from the “new” flu virus.