Castra literally means "castrates" or "emasculates." It's a very strong and somewhat shocking verb to encounter in a song, making it instantly memorable and intriguing.
In the lyrics, the phrase "me arrastra, me agarra, me amarra y me castra" (it drags me, it grabs me, it ties me up and it castrates me) is used metaphorically. The singer is expressing how the intense and overwhelming emotions of this "maldito verano" (cursed summer) and the person's gaze are completely stripping him of his will, power, and sense of self, leaving him feeling utterly helpless and diminished.
Caramelos de Cianuro's "Verano" [Summer] is a powerful look back at a past relationship, full of regret and the lingering pain of a broken heart. The singer remembers a time when love blossomed in a special place: "Iluminada madrugada luces / Del horizonte / Donde aprendimos del amor / Juntos en un mirador" [Illuminated dawn lights / On the horizon / Where we learned about love / Together at a viewpoint]. But that place, and the relationship, are gone: "Recuerda bien el sitio donde tú creciste / Porque ahora esa ciudad ya no existe" [Remember well the place where you grew up / Because now that city no longer exists].
The song is set during a sweltering summer, which seems to amplify the singer's torment. He's tired of the same old games, the "maldito verano / Que me tiene dolidas las manos / En mi cama sudando por ti / Yo no pienso vivir" [damned summer / That has my hands aching / In my bed sweating for you / I don't plan to live]. He's learned his lesson: "son los mismos engaños / Que ahora sé detectar con los años / Y la dura lección que aprendí / Es que no espero más por ti" [they are the same deceptions / That I now know how to detect with the years / And the hard lesson I learned / Is that I don't wait for you anymore]. Even though he still sees her in his dreams, "en blanco y negro como los perros" [in black and white like dogs], he's determined to move on.