Eren Yeager is the central character of Attack on Titan, the manga by Hajime Isayama that became a hugely popular anime series. The clip takes his name and adds the Japanese insult baka to a made up English taunt, turning a serious shonen hero into material for a childish joke. Short fan made anime lines like this spread quickly on TikTok, Discord and soundboards because fans instantly recognise the name. It remains popular as a quick, fandom specific roast that only lands fully if you know the show.