Can you tell me what can a flu shot suppose to do?

Flu shot: Flu shots like other vaccines expose your body to either a partially killed or killed virus which triggers your immune system to make antibodies to it. If you get exposed to the virus later, your body is prepared and can rapidly mount an attack against it either preventing you from getting sick or greatly reducing the severity of the illness.

Influenza vaccine: Help to prevent getting ill from influenza or minimizing the effects of illness.