“prescribed” / “Follow the label”
Prescribe means that a doctor orders a medicine for a patient, and follow the label means to do what the instructions on the label say. Here the pharmacist is confirming the doctor's instructions for Frank's medicine. The number of doses, the length of treatment, and taking it with food apply to this prescription; they are not rules for every antibiotic.
“With breakfast and dinner, ideally.”
Ideally means preferably, or in the best circumstances. It describes what is desirable without making it absolute: preferably with breakfast and dinner.
“...His what?”
Repeating only the surprising part of someone's words asks for clarification or expresses disbelief. His what? shows that the pharmacist is startled by Tina mentioning Frank's stitches.
“It's an antibiotic, Tina. Not a spell.”
An antibiotic is a medicine that fights bacterial infections, and a spell here is a magic chant. Because Tina recites the dosing rules as if memorizing a spell, Frank corrects her. This medicine is for the infected cut on his hand; it is not meant to remove the old stitches on his face. The punchline is the gap between Tina, who turns medical instructions into a magic ritual, and Frank, who seriously corrects her.