What you think it is: Just one day of the week.
What it really is: The most important day of the week to rest and eat bread rolls and enjoy the sacred silence so SHUT THE FUCK UP.
In case you didn't know: Sunday is not a regular day in Germany. On Sunday we have "Sonntagsruhe" ("sunday rest") which originates from the Christian "Sabbath desecration". Maybe you didn't expect Germany to be so massively Christian, but long story short: On Sundays all shops and supermarkets in Germany are closed, because Germans are not supposed to work on Sundays. Yes, this is protected by law. There are exceptions, of course. If you really need to buy something, you can get it at a gas station. Sometimes little kiosks are open on Sundays too. Mostly this happens in Berlin, where they are called "Spätkaufs" or "Spätis". Bakeries have to be open on Sunday mornings too, because we Germans LOVE to have freshly baked bread rolls on our Sunday breakfast. This is as sacred as the Sunday itself.
Also Sonntagsruhe means that all Germans want to do their grocery shopping before Sunday, of course. On every Saturday German supermarkets are packed with people who are buying groceries as if the world is going to end. It's even worse if a national holiday is approaching. This madness is called "Wochenendeinkauf" (weekend shopping).
Oh, and the "Sonntagsruhe" also means that to must not make any noise on Sundays, or else people are allowed to complain about you and you might have to pay a fine.