A negative split means running the second half of a race faster than the first half. It's widely considered a smart pacing strategy because it involves starting conservatively and finishing with something left in the tank, rather than fading late.
It's when your second half is faster than your first half, for example running the first 21.1 km of a marathon a little slower than the second 21.1 km. The opposite, slowing down in the second half, is called a positive split.
Starting slightly conservative protects against burning out too early, which is one of the most common ways races go wrong. Finishing strong also tends to feel better and hurt less than fading over the last few kilometres.
Hold back slightly in the first third of the race, even if it feels too easy, then settle into goal pace and allow yourself to speed up if you're feeling good in the final stretch. It takes discipline early on to avoid getting swept up in race-day adrenaline.
That's a positive split, starting fast and slowing down as the race goes on. It's the more common outcome for underprepared or overexcited starts.