A “population” here is one species in one place: one herd, one reef, one flock. The LPI takes roughly 36,000 of those, measures how the size of each changed since 1970, and averages the relative changes into a single figure: -73%. So it’s the typical trajectory of a monitored population, not a headcount.

What it doesn’t tell you: how many individuals, species, or populations were lost, or even what share are shrinking. Around half of the monitored populations are actually stable or increasing. The average is pulled down because declines tend to be larger in magnitude than gains, and because the index is sensitive to how long each population has been monitored (short, noisy series swing harder).

On “why not just sum the animals”: you can’t add individuals across species (a sardine and a rhino aren’t one unit), and any raw total gets dominated by the most numerous taxa, so you’d mostly be tracking a few exploding populations. Per-population relative change is the attempt to avoid that.

Whether it’s the best indicator is a fair fight, and the OP’s link (Our World in Data, Hannah Ritchie) is the clearest plain-language read on it. The catch is that the headline usually gets reported as a headcount, which is exactly what it isn’t.