Usually, vaccines contain a weakend version of the live virus.

However, with COVID-19 this wasn’t possible for two reasons:

1. Even a weakened version of the virus could still have harmed immunocompromised people.
2. The virus was mutating quickly, and there was a chance that the virus you were vaccinated with would be significantly different than the virus you’d later be exposed to.

Instead, the COVID-19 mRNA vaccine delivered the genetic information of the (fairly stable) spike proteins on the virus.

This genetic information provokes the body into an immune response, readying it for future exposure to the virus.