Poquititito is a playful and highly emphatic diminutive of "poco" (a little). While "poquito" means "a little bit," adding the extra "-itito" suffix intensifies the smallness, making it mean something like "a tiny, tiny little bit."
In the context of the song, the repetition of this word emphasizes a strong, almost desperate desire for just a minuscule amount more of something, making it very engaging and memorable.