Whether a headphone's earcups are sealed or vented — open-backs sound more spacious but leak sound; closed-backs isolate.
Open-back headphones vent the rear of the driver to the air. This typically yields a more open, spacious and natural sound with a wider sense of stage — which is why most audiophile and studio reference headphones are open. The trade-off: they leak sound both ways, so they're useless on a train and audible to anyone nearby.
Closed-back headphones seal the cup, providing isolation for commuting, offices and recording, usually at some cost to spaciousness and with a greater tendency toward bass build-up. Neither is "better" — it's about where and how you listen. Pair either with attention to impedance and a suitable amp.