mRNA stands for messenger ribonucleic acid. mRNA is a genetic sequence that provides the blueprint for something to be developed in a cell (generally a protein).
The new COVID-19 mRNA vaccine contains a small piece of genetic material created in a lab. This mRNA creates a blueprint for a small part of the SARS-2-Coronavirus called the “spike protein.” This is the part of virus that causes the immune system to recognize that it needs to make antibodies toward this virus.
The mRNA vaccine CANNOT cause an actual infection, as the only part of the blueprint is for the spike protein, and not for the actual virus itself.