Rechargable Batteries

I am mainly listening to prog using headphones, which are supplied by rechargable batteries. Well my headphones are 3,5 years old, and one recharge lasts for shorter time of listening than it used to be. I thought that before replacing headphones, which in due time will inevitably happen, I should replace the batteries. Obtaining headphones I got NiCd batteries AA 1,2 V 700 mAh. In stores I found batteries with only 800-2500 mAh. What's this mysterious mAh, capacity of energy stored after one recharge of another electric character (therefore it would be inadvisable to utilize other batteries).

I kindly ask your advice, since it is ten years from my last physics lesson.