Sarra is a popular Mexican slang word that means something is 'lame', 'crappy', or of poor quality. It's an informal but very expressive word that you'll hear in casual conversations.
In the song, the artist laments, "Y neta que sarra tener que mirarla de cerca" (And for real, how crappy to have to see her up close), referring to his ex-girlfriend's mother. The word perfectly captures the awkwardness and frustration of the situation, making it a great piece of authentic slang to learn.
This song is one of the most intense breakup anthems you'll ever hear. Fuerza Regida's singer isn't just sad, he's completely messed up over a girl who left him. He lays it all out, saying she left him with a wounded heart, "Traigo el corazón herido, me dejaste bien jodido" (I have a wounded heart, you left me really screwed up). He feels like he's lost himself, turning to vice and pushing his friends away for her.
He complains about how he worked to give her a fancy life, singing in English, "I pay your bills with my guitar / Drive you 'round town in a fancy car." But she still left, even after they made plans "de llegar hasta viejitos" (to grow old together). Now he's just lonely and dealing with the awkwardness of seeing his ex's mom, who looks at him with pity. The song mixes real pain with some very blunt and vulgar language, showing just how much he's struggling with his feelings: "I need ya, I want ya / I love ya, I hate ya."