There are several levels of "bans".
The lowest level is getting FSK 18, which means that it can be sold openly, but not to anybody who isn't 18.
The next one is being put on the index by the BPjM.
https://en.m.wikipedia.org/wiki/Federal_Department_for_Media_Harmful_to_Young_Persons
There are two lists of the index, one for porn and one for everything else. In both cases the decision means that it is illegal to have advertisements for those products, sell, lend, or otherwise make them accessible for persons under 18, or have it visible somewhere in a store. Games magazines are also not allowed to write reviews.
Companies usually produce cut versions to avoid that.
The last level, and the most extreme one, is a legal ban, which is a criminal process. That would happen if a game is glorifying violence to an extreme extent, encourages real life violence against people or contains propaganda for illegal groups. That's basically to ban games like concentration camp simulator and the like.
Owning and playing them is still allowed, but nothing else.
It is rare (far below one percent, a handful per year) as most games companies just sell cut products. Also the requirements for really getting banned are now more strict.
We can play most games just fine. Especially since there is often a way to get the soft-banned ones semi-legally from Austria, where usually an uncut German version is sold.
And of course there is a thing called globalization. If you really want something you can get it somewhere else.
