That is the server updating entity movements, positions, health-checks, hit-boxes, and redstone power states. That is the server working really hard because of someone's monsterously huge farm generating thousands of drops per minute, or someone with a redstone clock ticking away really fast as found in duplicating-glitch exploiters, or someone with an insanely active railcar system with major amounts of minecarts flying over activator and detector rails frequently, a massive pile of boats in one area, or someone on your server spawning mobs at high density.