High-fructose corn syrup (HFCS) is a liquid sweetener made by converting corn-starch glucose to a glucose-fructose mix — typically HFCS-55 (55% fructose) for soft drinks and HFCS-42 (42% fructose) for baked goods. It's chemically similar to table sugar (sucrose is 50/50 glucose/fructose) but is cheaper and dissolves more easily, which is why it shows up in nearly every shelf-stable processed sweet food in the US.
The American Heart Association does not single HFCS out as more harmful than other added sugars — the AHA's 25 g/day added-sugar limit for kids treats HFCS and cane sugar identically on a gram-for-gram basis. But HFCS earns extra parental attention for three reasons: (1) it's the dominant added sugar in cheap kid-marketed foods (fruit snacks, sweetened yogurts, granola bars, bread), driving overconsumption by being everywhere; (2) some 2010s research suggested fructose-specific metabolic effects (de novo lipogenesis, fatty liver) at high doses, though the clinical picture is contested; (3) it's a sign of an ultra-processed food product, which correlates with poor nutrient density independent of sugar content.
For practical shopping, the move isn't to obsess over HFCS specifically — it's to use the Added Sugars line on the Nutrition Facts panel as the primary filter. A product with 8g of HFCS and a product with 8g of cane sugar both contribute 8g toward your child's 25 g/day limit. That said, products that lead with HFCS in the ingredient list almost always have other ultra-processed-food markers (artificial flavors, dyes, low protein, low fiber), so HFCS is a useful "is this even real food?" heuristic.
Where it hides: many ketchups, BBQ sauces, granola bars, "fruit" snacks, bread (yes, bread), sweetened yogurts, chocolate milks, juice boxes, breakfast cereals, salad dressings, "honey-roasted" nuts, and most candy. Even some labeled "natural" foods include HFCS if they're not certified organic. Foods certified USDA Organic cannot contain HFCS.