Souvenir is a French word that has been adopted into Spanish, meaning a memento or keepsake. It's interesting because it's a direct loanword, making it immediately recognizable to English speakers, yet its use here is quite poignant.
In the song, León Larregui uses "un vil souvenir" (a vile souvenir) to describe how he feels reduced to a mere object, a reminder of a past relationship, rather than a valued individual. This metaphorical use of a common word in a negative context makes it particularly striking and memorable.
"Souvenir" is about watching a relationship become something to remember instead of something you're living. The singer admits he got scared of the distance between them and pushed too hard trying to fix it, and by the time she says she wasn't ready and it's too late to go back, all that's left of what they had is a keepsake: a piece of yesterday.
The chorus turns that keepsake bitter. He's told he's special, but it doesn't feel like love, it feels like being kept around as a souvenir. The line "there is no love when you always look for it outside of you" is the song's real point: he blames a love that depends on someone else to feel whole, rather than blaming either person directly.