A couple of sites you might find helpful...
The best way (IMHO) is to think of ISA as a baseline atmosphere. At sea level, it's 15C and a pressure of 29.92. Naturally, as you increase in altitude *above* sea level, the temperature will decrease, as will the pressure. If it's 15C at sea level, the temperature at, say, 30,000 feet will be lower than 15C, but the temperature at 30,000 will still be ISA if the sea level tempt is 15C. In other words, the actual temperature of ISA varies with, and is corrected for, altitude.
Wherever you see figures for ISA plus or minus something, it's telling you that the temperature is that many degrees warmer/colder than ISA. For example, ISA+10 means that it's 10 degrees warmer than ISA. As far as translating that into an actual temperature, one must know the altitude involved.
We use ISA as a more general consideration in flight planning, i.e. the amount of deviation in the airmass as a whole, since the aircraft will be operating at various altitudes on climbs/descents, as well as at specific crusing altitudes. Colder air, like ISA, or ISA-nn, is better than operating at ISA+nn as far as fuel consumption goes.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.