It is the maximum power dissipation in the crystal. This is one of the important specifications for a crystal. In operation, if the power dissipated in the crystal exceeds the specified drive level, the crystal may have long term reliability problems. The oscillation frequency may shift from the desired value, and in extreme cases the crystal may crack and stop oscillating altogether. Apart from this crystal should satisfy following requirement:
1) 24 MHz +100 ppm
2) Parallel resonant
3) Fundamental mode
4) 12-pF (5% tolerance) load capcitor.