Many Germans use slang in their everyday speech. Some slang may be rude or inappropriate, so it's best to avoid that. But there are lots of other kinds of slang expressions that are considered "normal" and perfectly polite in everyday speech.
Diese Frau ist einfach ein Dauerbrenner.
This woman is simply a long burning oven.
Caption 1: Angelique Kerber: Fotoshooting mit dem neuen Porsche 718 Boxster S
Calling somebody a "long burning oven" in English sounds a bit odd, to say the least, and possibly even insulting, but the slang German expression merely means that somebody is perennially popular.
Sophie, bist du immer noch sauer?
Sophie, are you still sour?
Caption 6, Die Pfefferkörner: Gerüchteküche
Of course, the person doesn't really want to know if Sophie is literally sour, but if she is still angry!
Weil du natürlich ihr Bärenführer wirst.
Because you will, of course, become her bear trainer.
Caption 36, Großstadtrevier: Von Monstern und Mördern
Nor does der Bärenführer have anything to do with real bears, what it means is a job trainer for new employees.
Frühlingszeit ist Fahrradzeit, also raus mit dem Drahtesel.
Springtime is bike time, so get out the wire donkey.
Caption 1, Fahrrad: Frühjahrs-Check
Naturally there are no donkeys, mules, burros or other pack animals involved, der Drahtesel is a slang word for bicycle!
Go to Yabla German and search for "slang" to find examples of slang German words used in videos. Then test the knowledge of other students in your German class by using the newly-learned German slang word in an appropriate context. For those who don't understand, it could sound pretty funny: imagine telling the class you rode your "wire donkey" to school that day!