Nachäffen means 'to ape' or 'to mimic' someone, usually in a mocking or childish way. It's a wonderfully descriptive verb, combining Affe (ape/monkey) with the prefix nach- (after), literally meaning "to ape after" someone.
In the song, Namika uses this word to complain about her partner's behavior, singing, "Vor unsern Freunden äffst du mich nach" (In front of our friends, you mimic me). It's a perfect, slightly cheeky word that vividly illustrates her frustration with his immaturity.
Ever felt like you are speaking a completely different dialect than your partner? In "Kompliziert", German singer Namika tackles the everyday comedy of couple miscommunication. She lays down the law right in the chorus: "Ich bin nicht kompliziert, du verstehst mich nur nicht" ("I'm not complicated, you just don't understand me"). She says "no," he hears "yes"; he knocks on the bathroom door asking if she's still alive while she fixes her hair; and he acts like the big boss in public ("Vor den Leuten machst du auf Chef" ["In front of people you act like the boss"]) even though he serves her breakfast in bed at home. It is blunt, funny, and painfully familiar.
Namika jokes that she needs to hand him a dictionary ("Wörterbuch") because her words sound like Chinese to him ("wie Chinesisch für dich klingen", the German idiom for saying something makes zero sense). But behind the teasing, there is real affection. Even when she admits she tunes out his excited monologues ("Dann tu' ich interessiert und hör' einfach nicht hin" ["Then I pretend to be interested and simply don't listen"]), she still accepts him as he is ("trotzdem nehm' ich dich so hin"). It is an entertaining track packed with real, conversational German slang about the chaotic reality of sharing a life.