Cursilería is a fantastic Spanish word for "cheesiness" or "corniness," perfectly describing anything that is overly sentimental or cliché in a romantic way.
In "Tanto," the singer admits, "Tanta cursilería, me suele incomodar" (So much cheesiness, it tends to make me uncomfortable). This is ironic because the whole song is an incredibly sweet and heartfelt declaration of love, showing just how much the other person has changed them.
Have you ever felt something so big that you couldn't find the right words to say it? That's exactly what's happening in this duet between Mexican duo Jesse & Joy and superstar Luis Fonsi. They sing as two people in a relationship who are both pretty bad at being romantic. She says he finds her "callada, reservada y temperamental" (quiet, reserved, and temperamental), and he admits that too much "cursilería" (cheesiness) makes him uncomfortable. They both know they aren't great at expressing themselves, but they agree, "Pero lo voy a intentar" (But I'm going to try).
And when they do try, they go big! The chorus is this huge, almost funny declaration of love. They sing, "Te amo tanto / Tanto que me siento tonta/tonto" (I love you so much / So much that I feel silly). To explain just how much they love each other, they come up with an impossible measurement: "Cuenta todas las estrellas y súmale una más" (Count all the stars and add one more). It's a super sweet way of saying their love is bigger than anything you can imagine. The song is a fun look at how even the most reserved people can find a way to express massive feelings, even if it feels a little goofy.