The marketing term fell out of favour; the underlying science — and the practical advice — didn't change.
Free radicals are unstable molecules produced naturally during normal metabolism (and increased by things like smoking, pollution and excessive alcohol) that can damage cells when they accumulate faster than the body can neutralise them — a state called oxidative stress, which is closely linked to, but distinct from, inflammation.
Antioxidants — including vitamins C and E, and many of the polyphenols covered in our polyphenols deep dive — neutralise free radicals, helping keep oxidative stress in check.
Antioxidant capacity was once heavily marketed using a lab measure called ORAC (Oxygen Radical Absorbance Capacity) — the USDA actually removed its public ORAC database in 2012, citing concerns that the values didn't reliably predict real health benefits in the human body the way marketing implied.
The practical takeaway hasn't changed even though ORAC marketing fell out of favour: a colourful, varied diet remains one of the more reliable ways to get a broad mix of antioxidant compounds — no need to chase a specific lab number.