Depending upon your daily affairs in Germany or things you might bump into.....the word verboten might get used a fair bit.
It basically means 'forbidden'. So it relates to a phrase which some Germans will utter: Das ist verboten. Meaning that's forbidden.
Not to be too cynical....but through the eyes of an American walking around....there are a fair number of things which are verboten.
Naturally, a German would quickly argue that point....noting speeds on the autobahn are usually unlimited, that sixteen year old kids can drink beer, and that nude swimming is allowed in a fair number of places.
Verboten things: bar-b-q's in city parks, passing on the right on the autobahn, carrying large sums of money across the border, letting your dog crap on a public street and you don't pick it up, driving a car barefoot, using the phrase 'du' when talking to a policeman, using the 'little-birdie-in-the-head' hand signal while insulting another driver, warming up your car for more than a minute on a cold January morning, etc.
To be honest, it's not a big long list of verboten things....it's just that Germans like to remind a non-German of the verboten things....in hopes that their new friend, associate, or lover.....won't screw up and commit a verboten thing.
When reminded of this verboten thing.....it's best to thank your girlfriend or associate, and just grin. You don't need to ask a lot of stupid questions or get into a forty-minute conversation about the verboten act. Just assume that the German culture spent years thinking about this, and came to a national consensus......this was something that needed to be forbidden.
Besides.....das ist verboten has a nice ring to it.