The iron triangle is the classic model of project constraints: time, budget, and scope, often stated as delivering "on time, on budget, to scope." The three are interdependent — tightening one typically forces a trade against the others — and together they define what project management has traditionally counted as success.
The limitation of the iron triangle is that it measures delivery, not value. A program can hit every constraint — on time, on budget, fully in scope — and still produce no benefit, because none of the three dimensions asks whether the delivered output is adopted or whether it moves the number it was meant to move.