R :
ISA stands for International Standard Atmosphere. It says that on an ISA standard day, the sea level pressure is 1013.2 Millibars / HectoPascals ( for the new boys ), the sea level temperature is 15degrees Celcius, and the air density is 1225 grams per cubic metre. Also, it assumes a temperature drop of 1.98 degrees Celcius per 1000' up to 36000 feet ( or thereabouts ).
Where it comes into play in aviation is mostly in aircraft performance. If the temperature increases, or the air is less dense, or the pressure decreases, all aircraft and their engines tend to perform worse than ISA, and if the temperature drops, pressure increases and density increases the opposite is true. It can be used by manufacturers to provide charts based on an ISA standard day, then appropriate corrections to the one table is all that is needed, rather than lots of tables for lots of different combinations of temp., pressure and density.
Is that a help ?
As to the Concorde stuff, are you there, WOK ?