At any given moment, your body is successfully fighting off and destroying millions of pathogens (bacteria and viruses) that threaten your system.
Each of us comes standard at birth with an incredibly lethal and startlingly accurate defense system called an immune system. It consists of an army of different types of specialized blood cells that form antibodies against invaders.
With traditional vaccines (those before the mRNA vaccine used against COVID), a "weakened" version of a virus is injected into the bloodstream, thus allowing the immue system to form antibodies to a less lethal version of a virus, and thus immunity, to that particular virus.
The vaccines used for SARSCov2, the virus that causes COVID, operate in a very different manner. With this vaccine, you are not being injected with a weakened version of the virus. In fact, you aren't being injected with any part of the virus whatsoever.
Instead, this vaccine uses what is called an mRNA molecule that actually causes your body to produce the foreign particles.
mRNA vaccines introduce synthetic mRNA into our bodies that then causes our own cells to produce a foreign virus component called a "spike protein." After those foreign spike proteins are produced, our immune system kicks in, recognizes them as foreign, and begins to make antibodies in response.
So the main difference is, earlier vaccines introduced weakened viruses to elicit an immune response, whereas mRNA vaccines actually "hijack" our body's own cellular protein replication machinery to produce the foreign components, that are then attacked by our immune system.
Philosophically, many, many people have a huge objection to this form of vaccine. Some people have even referred to it as "playing God." And I don't disagree.
Prior to the COVID pandemic, mRNA vaccines had never been used on humans. Add the fact the inventor of the mRNA vaccine technology, Dr. Robert Malone, has taken a firm stance against using mRNA vaccines in humans (especially in children) along with numerous other concerns that I won't address here, a strong case can be reasonably made for refusing these vaccines.
So there you have it!