Quietud translates to "stillness" or "tranquility." It's a beautiful and somewhat poetic word that evokes a sense of calm and peace.
In "Luna De Octubre," the lyrics describe how the October moon reflects "la quietud de dos almas" (the stillness of two souls). This suggests a profound sense of peace and harmony shared between two lovers, making it a more evocative choice than a common word for 'calm'.
"Luna De Octubre" builds itself around a single image and keeps returning to it: an October moon standing in for a love that has held. Lupita Infante opens by ranking it above every other moon there is, "La de octubre es más hermosa" [The one in October is more beautiful], then gives the reason in the very next line, "Porque en ella se refleja la quietud" [Because in it, the stillness is reflected], of two people who only ever wanted to be happy together. It's a strange, lovely way to open a love song, looking up at the sky instead of at the person you love, and it works.
The second half turns that memory into a vow. The heart she's singing to "Que has sabido sufrir" [That you have known how to suffer] "Y has sabido querer" [and you have known how to love] is the one she asks, whatever happens, not to let anyone disturb her golden dreams. By the last lines the song settles on one plain fact instead of a grand finish: she'll go on loving "Desde el día en que te vi" [from the day I saw you]. No twist, no new lover waiting somewhere else, just the same devotion the first verse promised, still standing.