Smearing is the perception of motion blur that reduces the sharpness and detail of the image in VR. Smearing along with strobing is part of the judder phenomenon in head-mounted displays. Because smearing reduces the visual quality of the display and even cause simulator sickness, it can ruin a user's immersion.
Smearing occurs when each pixel moves across the retina while it is lit. The longer the pixels are lit (full persistence) and the more movement of the HMD (quickly turning your head), the more smearing occurs.