A simple vista is a beautiful Spanish idiom that literally means "at a simple view," but translates to "at first glance" or "to the naked eye". It refers to what is immediately apparent without a deeper look.
In the song, Julieta Venegas describes an internal change that isn't obvious on the surface. She sings, "hay algo más que a simple vista no se ve" (there's something more that can't be seen at first glance), creating a sense of mystery and inviting us to look beyond appearances.
Julieta Venegas sets up a classic relationship dilemma in this song. Someone asks her, "Si nos podemos ver después" (If we can see each other later), but she can't give a straight answer. It's not that she doesn't have reasons; in fact, she says, "Razones me sobran" (I have more than enough reasons). The problem is a feeling she can't quite explain, something happening just under the surface. She repeats that there's "algo más / Que a simple vista no se ve" (something more / that isn't visible at first glance).
This isn't about forgetting the good times. She makes sure to say, "No pienses que olvido / Lo que me ha dado tu querer" (Don't think I forget / What your love has given me). But she can't ignore the central feeling of the song: "siento que hay en mí algo que está cambiando" (I feel that something in me is changing). It's a personal transformation that is invisible to others, but it's real, and it's getting in the way of simply saying "yes" to meeting up. She wants happiness and love, but first, she has to figure out this change within herself.