Rebuscar means "to rummage" or "to search thoroughly," often implying a desperate or exhaustive search. It's a more intense and specific verb than simply "buscar" (to look).
In the song, Manu Chao sings, "Cada día me espanto de tanto rebuscar" (Every day I'm scared of so much searching/rummaging). This suggests a feeling of being overwhelmed or exhausted by constantly digging for answers, meaning, or perhaps even survival in a difficult world. It paints a vivid picture of a relentless struggle.