It ‘injects’ RNA into your cells that causes them to make part of a virus (parts of the outside of it, not the parts that make it reproduce).

Those parts, in turn, trigger your immune system to build up defenses against the virus so that, if it encounters the real virus, it can contain the infection.

mRNA degrades fairly rapidly, so this won’t keep triggering your immune system (but https://en.wikipedia.org/wiki/RNA_vaccine#Side_effects_and_r... says the mechanism used to do the ‘inject’ part may trigger its own immune reaction)