Yeah - I know this one! The "12 volts", as you almost certainly know, refers to the voltage of the battery, which is like a measure of the electrical pressure that it can push out. SLAs are most often found in 2v, 6v or 12v sizes. The second figure shows the capacity of the battery, or its ability to store and deliver electricity. The value is shown in Amp Hours, or the number of amps it can supply in 1 hour. Thus a 1.2 Ah battery can supply 1.2 amps for an hour, and a 7 Ah one can supply 7 amps for an hour, or 1 amp for 7 hours, or any combination of the two which multiply to give 7. That means that, for example, if you know your motor will draw 2 amps on load then a 12 volt 7 Ah battery will last for 7/2 = 3 1/2 hours. It's just the same for NiCad and NiHM packs, except that convention is to quote their capacity in mAh.
OK?
HTH
FLJ