Reacio means 'reluctant' or 'hesitant'. It's a less common word than its synonyms, making it stand out.
In the song, Nicky Jam sings, "Contigo un poco reacio" (With you a little hesitant). He uses this word to describe his initial reluctance or hesitation in the relationship, perhaps playing hard to get or being cautious, which adds an interesting layer to the romantic pursuit in the lyrics.
Nicky Jam spends this whole song confessing things he opens by insisting he never confessed. "Yo no te he dicho cuántas veces yo he soñado con tu cuerpo" (I haven't told you how many times I've dreamed about your body) is the first line, and the second is no more subtle, since her perfume is apparently still sitting in his apartment. The hook is one man asking the same question on a loop, "¿Cuándo viene a verme?" (When are you coming to see me?), then answering it himself with "Es que estoy loco por verte" (I'm going crazy to see you). There is not much mystery to the plot.
What saves it from pure flattery is how annoyed he sounds about his own situation. "Te estaba dando tu espacio" (I was giving you your space) he says, as though playing it cool had been the plan, before abandoning it for "Tú eres el mal que pa' mí es necesario" (You're the evil that's necessary for me). The best moment is the shrug that closes the verse, where he hands the blame to somebody else: "Le corro al amor, pero él me persigue" (I run from love, but it chases me), and then "Cupido te mandó pa' que yo te castigue" (Cupid sent you so I could punish you). Pinning your own bad decisions on Cupid is a very reggaeton way out.