My understanding, although I am not a doctor, is that it boosts your immunity to the virus itself because they inject you with a active (but very weakened) version of the virus. That way when your immune system is depressed, its response is still much better than it would have been otherwise. More antibodies produced, less chance the dormant virus can reactivate. Really interesting, honestly.
It's more complicated, there are many variants. Some use a live weakened version of the virus/bacteria. Some use a dead versions. Some use only a small part of the surface of the virus/bacteria that is enough to be recognized. (The last is similar to the approach in this vaccine.)
[In some old vaccines, they used a somewhat related similar virus/bacteria that was close enough to make you get the immunity, but not to cause the illness. I'm not sure if this method is used still today.]
