Hi-Cap magazines are airsoft magazines with a larger volume. On average you can do 300/350 BBs in a Hi-cap magazine before you have to top up. There are even certain hi-cap magazines (For AK, etc.) where you can store 500 or more BBs.
With turning wheel or pull cord?
There are 2 types of Hi-cap airsoft magazine. One is equipped with a gear at the bottom, the other with a pull cord (this is called Flash magazine). In both cases you will have to tighten the airsoft magazine every 20/25 bb's to continue playing.