The specs simply mean that at 25Hz, the sub plays at -3dB efficiency compared to the average efficiency over, say, 30-80Hz; and at 18Hz, the efficiency goes down to -9dB of average. Efficiency is the sound level for a given signal level. So, the sub "goes" down to 25Hz and to 18Hz but with different efficiencies. Keep in mind that -3dB represents a half physical power of the sound, and -10dB denotes that the sound is about "half as loud subjectively" to human ears.
Another important measure of a sub's performance is the maximum peak sound level it can reproduce at given bass frequencies (for example, 110dB at 25Hz).
