Thermal mass is the capacity of a material to absorb, store, and release heat. High thermal mass (concrete, stone, earth) smooths temperature swings — cool spaces stay cool longer, warm spaces stay warm longer.
In passive solar design, thermal mass positioned to receive winter sun charges during the day and releases heat at night. In hot arid climates, thick earthen walls absorb daytime heat that would otherwise enter the interior.
Modern "energy-efficient" buildings often have low thermal mass and high insulation — they're good at preventing heat transfer but poor at buffering temperature swings. The result: HVAC systems have to work hard to compensate.