That kind of interrupting rating means it clears a bolted fault cleanly, keeping downstream devices alive and the rest of the bus running.
Thermal derating — real-world current
Rated 125 A continuous at 40 °C, but the frame derates as the panel warms: 122 A at 55 °C, 120 A at 60 °C, 117 A at 65 °C, and 114 A at 70 °C. If your enclosure runs hot, size the load side for the derated number — the breaker won't nuisance-trip on a warm day.
Dimensions are 130 mm high, 50.8 mm wide, 70 mm deep — a compact 2-pole footprint that fits standard 50 mm wide MCCB slots.