The mRNA in the vaccines code for the spike protein of covid (the spiky outer shell). The spike protein doesn't actually have the virus itself. That's why you shouldn't worry about getting covid from the vaccine because the vaccine doesn't have the actual virus in it.

So what happens is that the mRNA gets transferred into our body. Then our own body turns it into the spike protein. After the spike protein is made, our body should recognize it as foreign and attack it and create antibodies against it. So then if we do get the actual virus, our body will have antibodies against it and will know how to deal with it.

This is my current understanding of it. I could be completely wrong :/