Immunisation provides immunity to communicable diseases. This is achieved by injecting or administering orally, small amounts of dead or weakened antigens into the body. This is called vaccination. The body is stimulated to produce antibodies. Often a second booster injection is give to stimulate a faster and longer-lasting production of antibodies.
When you catch a disease your body makes antibodies to kill the pathogens that are what is actually passed from one person to another. Immunisation is a way of giving your body some immunity by giving it some previously killed pathogens so that they cannot harm you but your body can make antibodies that protect you from the real thing. This is not the only way it can be done but it is the most common mechanism, much simplified.