Immunisations are injections we have to protect us against catching certain nasty diseases. These diseases include meningitis, whooping cough, polio, measles, mumps, and several others. These diseases can be very dangerous, and can even kill you, so immunisations are a good thing!
How immunisations work
The immunisation that is injected into you is entirely safe and will not harm you in any way but is made made in such a way that it gets your own body to produce antibodies against the disease that it protects you against. So when you have a 'tetanus'immunisation injection, you are injected with a substance which is not actually harmful to you in any way but makes your body think that it should produce antibodies against tetanus, so that if you really did then get exposed to tetanus then your body is ready to deal with it.
You will have had most of your immunisations when you were a baby or a toddler, so you won't remember them. However, you will have also had some top-up (booster) injections when you are at school, to make sure you were protected against the diseases. Injections are a little bit uncomfortable, but nothing more than that, and certainly worth having because the diseases they protect you against are a lot worse!