Preseas is a conjugated form of the Puerto Rican slang verb presear. It doesn't have a direct translation, but it means to show off, to posture, or to act jealous in a provocative way, especially when you no longer have a right to.
In the song, Daddy Yankee repeats the line "¿por qué preseas?" ("why are you fronting?"). He's calling out a woman who is already in a new relationship, yet gets jealous and makes a scene when she sees him with someone else. It's a unique and powerful slang word that perfectly captures the song's central conflict.
This song is a classic break-up anthem from the Puerto Rican superstar Daddy Yankee. He's telling a woman that whatever happened between them is firmly in the past. The reason is simple and direct: "Esa noche contigo la pasé bien / Pero yo me enteré que te debes a alguien" (I had a good time with you that night / But I found out you belong to someone else). He found out she was cheating on someone to be with him, and for him, that's a deal-breaker.
He doesn't hold back, calling her an "engañadora que te envuelve y te domina" (a deceiver who wraps you up and dominates you). Now that he's moved on, he notices she's jealous and possessive, even though she's the one with a partner. He taunts her, saying, "Ahora estás celosa porque otra me desea" (Now you're jealous because another woman desires me). The song's title becomes his final word on the matter: "Lo que pasó, pasó" (What happened, happened). It's over, and he's not looking back.