Manantial translates to "spring" or "fountain" (as in a natural water source). It's a beautiful and somewhat poetic word that evokes images of purity and life.
In "Fresa Salvaje," Camilo Sesto uses manantial to describe the woman he's singing about, calling her "Agua de manantial" (Water from a spring). This metaphor suggests she is refreshing, natural, and a source of life, adding a layer of depth to his admiration for her.
This ballad uses a wild strawberry as an image for a woman who is untamed and alive, growing free rather than cultivated. The singer follows her like water finding its own path, promising quiet, loyal devotion (the faithful dog is his own comparison for that loyalty) in return for her presence in his life.