A number set by length, mostly
Par is assigned to a hole largely according to its length, following broad yardage bands that expect a certain number of shots to reach the green plus two putts. This means par is fundamentally an estimate based on distance, not a direct measurement of how difficult a hole actually plays, which is why organisations occasionally reassign par to a hole after redesign work even when the challenge it presents has barely changed.
Same par, different difficulty
Two holes can carry an identical par while playing at very different difficulty levels, because par ignores hazards, green complexity, wind exposure and the shape of the fairway entirely. A short par four guarded by water and a firm, sloped green can easily prove harder than a longer par four with a wide fairway and a flat target, even though the scorecard treats them as equivalent tests before a single shot is struck.
Why the convention still works
Despite these limitations, par remains useful precisely because it is a shared, simple convention: it lets scores be compared meaningfully across entirely different courses, which a raw stroke total alone could not do. Its value is comparative rather than descriptive, and understanding that distinction explains why professionals rarely treat par itself as a target, instead judging a hole by its actual demands and simply using par as the common language to report the result afterwards.