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Not enough fluid in a hydraulic power steering system, so the pump cannot maintain pressure. The pump depends on fluid for cooling and lubrication as well as for pressure, so running it low destroys it — which is why this matters beyond the steering feel.

Aeration is the part that produces the odd symptoms. A level low enough to let the pump draw air gives a system that whines, feels lumpy, and is worse on lock and when cold — and the behaviour changes with cornering and gradient as fluid moves in the reservoir. Foam in the reservoir is the confirming evidence.

These are sealed systems, so a low level always means a leak. Topping up without finding it means meeting the same fault again with a pump that has been damaged in the meantime. Check the rack gaiters, the hose crimps and the pump's shaft seal, and use the specified fluid — many systems require a particular type, and mixing them degrades the seals that were holding.