'Brightening', 'radiance-boosting', 'glow-enhancing' — these are marketing terms that mostly describe products containing vitamin C, niacinamide, alpha arbutin, or tranexamic acid. Reading ingredients matters more than the marketing claim.
The ingredient inventory of most brightening serums
L-ascorbic acid (vitamin C) — most common, well-evidenced. Niacinamide — supports barrier, mild brightening. Alpha arbutin — mild tyrosinase inhibitor, good for hyperpigmentation. Tranexamic acid — best evidence for melasma specifically.
How to pick what actually matches your need
General prevention + antioxidant: vitamin C. Post-acne marks: alpha arbutin or azelaic acid. Melasma: tranexamic acid. Overall radiance: niacinamide for barrier + sleep + sunscreen do more than any 'brightening' serum.
Read the active ingredients. Three brightening serums are usually the same vitamin C in different packaging.