ISA+X is the ISA atmosphere with X°C of variation.
It´s used to determine how diferent (in terms of Temperature) of the Standard the actual condictions are and adjust the calculations.
ISA as you said is:
+15°C / 0ft / 1013.2 hpa / LAT 45° / Term. Grad. -2°C/1000ft
So, ISA +5 is:
+20°C / 0ft / 1013.2 hpa / LAT 45° / Term. Grad. -2°C/1000ft
The pratical use is like the following:
If you are at FL075 and the OAT is -15°C whats your difference to ISA?
ISA Air Temp. @ FL075 is:
ISAt = 15 - 2 * alt/1000
ISAt = 15 - 2 * 7500/1000
ISAt = 15 - 2 * 7.5
ISAt = 15 - 15
ISAt = 0°C
So you are 15°C below the ISA temp at your FL or simply ISA-15.