Fantaseo means "I fantasize" or "I dream". It's an engaging word that immediately hints at a deeper, more imaginative side of the song's narrative.
In the context of the lyrics, the singer admits, "a veces fantaseo con tener el control" (sometimes I fantasize about having control). This reveals a vulnerability and a desire for power that is contrasted with the song's central theme of "ilusión óptica" (optical illusion), making it a key word for understanding the song's emotional core.
"Ilusión Óptica" is Belén Aguilera looking in the mirror and not trusting what she sees. She wonders what people do when nobody's watching, and admits she fantasizes about being in control even though she knows that feeling is fake, just an optical illusion.
The chorus keeps circling back to the same idea: cover it up, that's not really me. She's more comfortable being imagined than being seen, letting someone picture her instead of showing the real, less tidy version.
By the final verses her self-esteem is on the floor while her ego stays sky-high, and she admits to feeling jealous of people who seem to handle their own grief better than she does. It's a song about the gap between who you look like you are and who you actually are underneath.