In photography, to let too little light reach the film or the sensor, so the picture comes out too dark. The shutter closed too fast, or the room was darker than you thought, and the shadows turn into flat black with nothing in them. You can underexpose a photo, or a photo can be underexposed — both are normal. The opposite is 'overexpose', where too much light makes everything pale and washed out. Photographers today sometimes underexpose on purpose: a digital sensor can often bring back detail from dark areas but never from a blown-out white sky, so 'expose for the highlights' means deliberately shooting a little dark. So the word is not automatically a criticism, though on its own it usually points at a mistake. Occasionally you meet it outside photography, meaning someone got too little exposure to something — an idea, a language — but that use is uncommon and 'have little exposure to' is what people normally say.

曝光不足（照片太暗）。攝影術語，相反是 overexpose 過曝。