Rhesus negative

Hi. Is anyone here same blood type like me? I’m rh negative(O-) just now i just had my 28weeks injection named Rhogam. But the doc did not explain about why i should have this injection and how the injection works actually. I went to gov hospital and there is a lot of people waiting after me so I guess that is why they have no time to advise me anything. ??‍♀️