Piénsatela is a colloquial and emphatic way of saying "think it over" or "think about it carefully." It's a combination of the verb pensar (to think) and the reflexive pronoun te (you), plus the direct object pronoun la (it).
In the song, the artist uses piénsatela dos veces (think it over twice) to advise someone to consider their decision very carefully, especially regarding the risks of falling in love. It's an engaging word because it's not just a simple command, but a more personal and intense urging to reflect.
"Soy Lo Que Ves" is a quiet confession from someone who would rather be honest than say what the other person wants to hear. He can listen, he can care, but he's upfront that he can't promise to stay or to fall fully in love — that plainness is what he calls his religion, the truth over comfort.
The chorus turns to a very modern kind of heartbreak: being left on read. He knows the risk of being this honest is losing her, and he's willing to take it anyway, even as it slowly wears him down.
It's a song about choosing sincerity over a performance of love, and living with the loneliness that choice can bring.