Zeroing and Bullet Drop
Every bullet in PUBG drops over distance due to gravity. Zeroing adjusts your scope to compensate at a specific distance. Press Page Up/Down to change zeroing. At 300m zero, your crosshair aims where the bullet will land at 300m. For most engagements, learning to aim above the target (Kentucky windage) is faster than changing zeroing mid-fight.
Leading Moving Targets
Bullets take time to travel. A Kar98k bullet reaches 300m in about 0.38 seconds. A running player covers about 6m per second. That means you need to lead a running target by approximately 2.3m at 300m (about one full body width ahead of the direction they are running).