Anis is the Spanish word for "anise", a spice known for its distinct, licorice-like flavor.
In the song, Shakira sings of saying goodbye to a "boca de anis" (anise-flavored mouth). This is a beautifully poetic and unusual metaphor for a kiss, suggesting something that might be sweet at first but leaves a sharp, lingering, and perhaps even unpleasant aftertaste, perfectly capturing the bittersweet feeling of leaving a lover behind.
In this classic pop-rock track, Shakira is packing her bags and leaving a lover behind, using the city of Madrid as a stand-in for the entire relationship. She's decided it's time to hide her pain—"esconder / Del mundo el dolor / Bajo la piel" [to hide the pain from the world under the skin]—but she's not worried. She knows she'll be fine because, as she puts it, "Los gatos, como yo / Caen de pie" [Cats, like me, land on their feet].
She's leaving because the relationship has become superficial and unstable. She calls out her ex's "rutinas de piel y tus ganas de huir" [skin routines and your desire to flee], suggesting someone who was more about appearances and always looking for an escape. Shakira is having none of it, declaring bluntly, "Yo no quiero cobardes que me hagan sufrir" [I don't want cowards who make me suffer]. So, she says goodbye to his "boca de anis" [anise-flavored mouth] and gets ready to move on.