Incertidumbre translates to "uncertainty". It's a sophisticated and emotionally charged word that captures a complex feeling, making it more engaging than common vocabulary.
In the song, the artist uses it to express the deep doubt and anxiety of not knowing if his lover's affections are truly for him, singing, "Incertidumbre de pensar que tus besos no son para mí" (Uncertainty of thinking that your kisses are not for me). This word perfectly encapsulates the emotional core of the lyrics.
"Camina Sola" is about a relationship that keeps breaking and mending. The narrator describes the wounds she leaves him with, and how he can't help being drawn back to her anyway.
The chorus paints her walking down the street alone, and his house feeling sad and empty because she's not there. Despite the hurt, he admits he doesn't know whether to give her everything or walk away for good, and that the on-and-off pattern between them keeps repeating.