What it means
To approve a generic, the FDA requires it to have the same active ingredient, strength, dosage form and route as the brand, and to be bioequivalent. Usually that is shown by measuring blood levels in volunteers who take the generic and the brand.
Bioequivalence does not mean identical in every way. Inactive ingredients, color and shape can differ. The FDA still requires the generic to work the same way and give the same clinical benefit.
Peptide generics such as liraglutide also need extra checks. The FDA expects makers to show the active ingredient is the same and that impurities do not raise the risk of immune reactions.
Compounded products are not tested for bioequivalence or reviewed by the FDA before sale. A compounded product that claims the "same active ingredient" as a brand has not been shown to act the same way in the body.
Why it matters
Bioequivalence is the line between an FDA-approved generic and a product that only claims to be the same.