It's the amount of current the battery can provide at 0 degrees F.
Although it doesn't get massively cold in Phoenix, CCA is still critical
because it gives an indication of what's apt to happen when it's 120 deg
in the shade. The Reserve Capacity is a good number too. I like to see at
least 40 minutes. That's how long your engine will operate off the battery
alone it the alternator dies. (No lights, heater or other drains at the time)
I've got an Optima red top in mine. (8020-164, Battery Council International
case style 35) I love these things. I've got 3 of my 5 cars outfitted with
Optimas. I got seven years out of the last one I "wore out". It still worked
fine but the starter had begun to slow down in the winter.
They are heavy muthas though, the one in the WRX is around 34 pounds.
It is more than the OEM Yuasa (or whatever it was) by at least 5 lbs.