Back in the olden days batteries were rated at an 8 hr discharge rate. That meant a battery could supply X amps for 8 hours.
As batteries changed and the demands on batteries changed different discharge rates were applied.
In general the higher the current draw the fewer "amps" can be drawn from a battery.
So 1A for 12H means the battery will provide 1 amp for 12 hrs. There are some caveats, first this is dependent on temperature, it is specific, used to be 80 F. Second, the battery will not provide 12 amps for 1 hr, but it may provide .5 amps for 30 hrs.
As for the meaning of "/2HR" or "/10HR" is there any other nomenclature?