The mRNA vaccine works the same way a vaccine with weakened viruses work. The difference is that not a virus hull is used to bring the RNA into the cell, but a fat coating. Once inside the cell, the RNA is used by the protein factories as a blueprint. Then, the cell detects the alien protein it just has produced, moves it to the hull, and present it to the immune system cells. The immune system cells start the learning process to fight the protein in the future (immunisation), and they destroy the cell (before more virus stuff gets produced).