Guppy is giving birth to yellow balls?

Guppies, like other livebearers, give birth to live young rather than eggs. The "yellow balls" you mention could refer to the fry (baby guppies) that are small and may appear yellowish depending on the parent's coloration and genetics. It's important to ensure that the mother is in a safe environment during and after giving birth, as fry are vulnerable to being eaten by adult fish. Ensure the fry have adequate hiding spots to thrive.