How and where your foot lands when running — heel, midfoot or forefoot. What matters more is where it lands relative to your body.
Few topics in running attract as much nonsense. The research is surprisingly clear: no footstrike is universally superior, and forcing a switch — say heel to forefoot — mostly relocates load rather than reducing it (away from the knee, onto the Achilles and calf), and is a common cause of overuse injury. The genuinely relevant point isn't which part of the foot lands first, it's how far ahead of your centre of mass it lands. Land far out front with a straight leg and you brake on every step. The most effective lever against that isn't at the foot but at cadence: a few percent more step frequency pulls your landing back under your hips by itself.