Coqueta is a fun, descriptive word for a person (usually a woman) who is playful and flirtatious. It describes someone who enjoys charming others, but might be a little mysterious about their true feelings.
In the song, the singer directly addresses the girl he likes as 'coqueta'. He's captivated by her, but also a bit frustrated, asking her to be direct and tell him the truth ('la neta') about whether she likes him back. It's the key word that captures the playful, uncertain energy of this budding romance.
This song is all about being completely obsessed with someone after a whirlwind romance. The singer is looking up at the stars, wondering if their connection was a cosmic event or just an "error del universo" (a mistake of the universe). He's desperate to relive their last encounter, pleading, "Baby bésame, quiero que vuelvas a mis brazos / Pa' sentirnos eternos, como la última vez" (Baby kiss me, I want you to come back to my arms / To feel eternal, like the last time). He's convinced she feels the same way, even though it all happened incredibly fast.
He directly calls her out for being a flirt, singing, "Coqueta, si me quieres sólo di la neta" (Flirt, if you like me just tell me the truth). He's not playing games and wants a clear answer so "que se entere todito el planeta" (the whole planet can find out). To prove he's serious, he makes a very modern and bold promise: "de mi cel ahí te va la clave, que yo no hablo con nadie, nomás contigo" (here's the password to my phone, I don't talk to anyone, only you). It's a passionate and impatient plea for her to stop being coy and just be with him.