Imperial immediacy is like being a special big kid on the playground who can make decisions all by themselves without asking the teacher.
In the olden days, all the people who lived in Europe were ruled by kings and emperors who told them what to do. But some of the bigger, stronger groups of people got tired of being bossed around by the kings and decided they wanted to be in charge of themselves.
These groups were called princes, and they were so special that the emperor said they didn't have to listen to the kings, either! They could make all their own decisions and be totally in charge of their own land, just like the emperor was in charge of the whole country.
This is what it means to have imperial immediacy - it's like being a really important prince who gets to make all the choices for his own land. It was a big deal back then because it meant you were really powerful and didn't have to answer to anyone else.