Ah is an abbreviation for ampere-hour, or amp-hour. This the total amount of charge a battery can deliver in one hour. A power tool that continuously draws 1.0A of current will completely drain a 1.0 Ah battery pack in one hour (under ideal conditions). Obviously this means that a 2.0Ah battery pack can power the same tool longer than a 1.0Ah battery pack can, assuming the current flow remains at 1.0A and there are no other differences.