On paper, a balanced three-phase load returns zero current through the neutral. In practice, the neutral almost always carries something — and it can come from two genuinely different mechanisms that behave nothing alike.
In a perfectly balanced three-phase four-wire system with purely linear loads, the three phase currents are equal in magnitude and evenly spaced 120° apart — and their vector sum is exactly zero. That's the textbook answer to "what does the neutral carry," and it's correct as far as it goes. Real systems, though, almost never sit at that ideal. Neutral current shows up for two distinct reasons that get conflated constantly: plain load imbalance, and harmonic distortion from nonlinear loads. One behaves exactly the way intuition suggests. The other doesn't — see the harmonic-current mechanism below for why. For the full three-phase background, see the three-phase power systems guide.