Dímelo is the command form for 'tell me'. It's actually three words fused into one: di (tell), me (me), and lo (it).
This structure of attaching pronouns directly to commands is super common in Spanish and is key to sounding more like a native speaker. In this salsa classic, Marc Anthony uses this powerful command as a passionate plea, urging his love interest to finally confess their feelings for him.
Have you ever heard a rumor that your crush likes you back? That's exactly where Marc Anthony is in this song. He's heard through the grapevine that someone he's into wants to get closer, singing, "La gente anda diciendo por ahí / Que tú quisieras acercarte a mí" [People are saying out there / That you'd like to get closer to me]. Instead of playing it cool, he goes all in, admitting his heart is "enloquecido con esta pasión" [driven crazy with this passion].
This whole song is one big, passionate plea for confirmation. He's not waiting around; he needs to know now. He begs her, "Si es que me deseas nena dímelo" [If you desire me, baby, tell me], because the feeling is so intense that "no me deja dormir" [it doesn't let me sleep]. The chorus is a simple, desperate demand: "Ay dímelo, ven dímelo" [Oh, tell me, come on, tell me]. He's putting all his cards on the table.