Merced literally means "mercy," but in the phrase a su merced it conveys being "at someone’s mercy" or completely under their control.
Quevedo sings, "me usa a su merced" (she uses me at her mercy), playing with the homophones USA and usa to show how the girl both studies in the USA and "uses" him however she pleases. It’s a poetic, somewhat archaic word that rarely appears in modern lyrics, making it stand out and perfect for your Spanish vocabulary list.
This song is the perfect soundtrack for a summer romance you know has an expiration date. Quevedo tells the story of meeting a girl who's just come back to the island after studying at Columbia University in the US. She's determined to have a fun, carefree summer and 'solo quiere salir y de nadie depender' (only wants to go out and not depend on anyone). That plan goes out the window when she meets him.
Their connection is instant and intense, even though they both know it's temporary. He sings, 'Bebé, los dos sabemos que es verano y que tal vez / Cuando termine agosto no nos volvemos a ver' (Baby, we both know it's summer and that maybe / When August ends we won't see each other again). Despite this, they fall for each other. She even accidentally lets an 'I love you' slip out, the girl who 'no quería nada' (didn't want anything). He looks back on all their shared moments, from holding hands on the beach to her singing his songs, calling it 'algo tan sano' (something so healthy), hoping it all meant something.