Aparentas comes from the verb aparentar, meaning "to pretend" or "to feign". It's a great word to learn because it describes putting on a front or acting a certain way that isn't true to your feelings.
In the song, the lyrics say, "Y que aparentas que te sientes bien" (And that you pretend that you feel good). This highlights the ex-partner's attempt to hide their true emotions after the breakup, making it a key word for understanding the song's emotional core.
The narrator has already done the arithmetic. "Sé que terminamos, eso ya lo sé" (I know we broke up, I already know that) is the first thing he says, and the rest of the verse is him watching her work around it — pretending she feels fine, asking people whether he is with someone, whether he is happy. He gives her one line back: "No te preocupes me encuentro muy bien" (Don't worry, I'm doing very well).
The chorus turns on a piece of everyday Spanish. "Ahora soy yo quien no te da la hora" (Now it's me who doesn't give you the time) — dar la hora is giving someone the time of day, and withholding it is how Spanish says you will not acknowledge a person at all. Under it sits the line that closes the door: "Se me acabó lo que guardé para ti" (What I kept for you ran out). Nothing was thrown away or taken back; it was simply used up. The one concession comes in the middle verse — "Me dolió cuando tú me dejaste" (It hurt me when you left me) — answered immediately by "Pero me duele más volver a perdonarte" (But it hurts me more to forgive you again), which is the whole case for staying gone.