The Immune System is a system that many people took for granted and then, in March of 2020, people started paying a bunch of attention to it. There's a simple version and a complicated version to the Immune System and how it works.
Simplified, the immune system is the main system that provides an immune response to "invaders" of the body, most often pathogens (germs). In more detail, you actually have two types of immune system: the "inate" immune system and the "adaptive immune system.
The "innate immune system" you're born with. It's more or less the same for everyone minus a few variations due to DNA or Pre-Natal care.
The "adaptive immune system" is a specific individual'sresponse by their body to pathogens they've been exposed to. The most common reference to this is when the body creates "antibodies" after being exposed to viruses of the common cold.