IBU (International Bitterness Units) measures dissolved iso-alpha acid concentration in finished beer, defined at roughly 1 IBU per milligram of iso-alpha acid per litre. It's a chemical concentration measurement, not a direct taste survey — a useful distinction, since two beers with an identical IBU number can taste noticeably differently bitter depending on malt sweetness, carbonation, and other factors entirely outside the IBU calculation.
Two formulas, genuinely different numbers, same input data
Homebrewers most commonly reach for one of two estimation formulas — Tinseth or Rager — that predict IBU from a hop schedule's weight, alpha acid percentage, boil time, and boil gravity. The two formulas routinely disagree, and not always in the same direction — Rager can read noticeably lower than Tinseth on a mid-boil addition and noticeably higher on a long, high-gravity one; see Tinseth vs Rager: why they disagree for the worked numbers behind that swing. This site's IBU calculator reports both rather than picking one as definitively correct.
Why BU:GU matters alongside the raw number
A raw IBU figure means little without context from the beer's gravity — the BU:GU ratio (bitterness units to gravity units) normalizes for strength, letting you compare how aggressively hopped a light beer is against a strong one on a level footing rather than by IBU alone.