Tremenda literally translates to "tremendous," which can describe something huge, terrible, or shocking. However, in this context, it's popular slang for "amazing," "awesome," or "stunning."
In the song, Juan Magan sings, "Tremenda como en el día que la conocí" (Amazing, just like the day I met her). He's using this powerful adjective to describe how incredible the woman he's watching is. Learning this colloquial use is a fantastic way to make your Spanish sound more authentic and natural.
Spanish producer Juan Magán brings peak nightlife drama in Bailando Por Ahí. The narrator is out in Madrid when he spots his ex-girlfriend hanging out with her friends on the street: "Ayer la vi bailando por ahí / Con sus amigas en una calle de Madrid" ("Yesterday I saw her dancing around / With her friends on a Madrid street"). Instantly, all his old regrets flare up. He remembers that their time together held his happiest days, and to make things worse, she looks just as stunning as the day they met ("tremenda como en el día que la conocí"). He puts on a tough act at the bar, orders a drink, and eventually walks over to flirt by touching her hair and offering to follow her on Twitter ("Te sigo en el Twitter si quieres te escribo"), which dates the scene hilariously to the early 2010s social media era.
Then things get messy. Her current boyfriend suddenly appears: "De repente tra tra llegó su novio ya" ("Suddenly, bam, her boyfriend arrived"). Instead of backing down, Magán dishes out some unsolicited relationship advice, warning the guy that couples grow apart in daily routines and telling him to step aside if he will not treat her right ("Si no la vas a cuidar échate pa'atras"). He gives the boyfriend a stern glare to make him sit down, but promises he will walk away if the girl really does not want him. Still, deep down, he is convinced she still wants him back. It is cheeky, boastful, and full of the chaotic energy of bumping into an old flame while partying in Spain's capital.