I am a new driver and I know nothing about cars.

what is the meaning of JDM WRX , AVCS and higher octance gas??

Well... JDM WRX means "Japanese Domestic Market" WRX (or course the subaru Impreza WRX)

AVCS - no clue

higher octane gas typically allows for higher compression ratio's (combustion chamber uncompressed/compressed volume ratio)

it's needed to avoid detonation in these high compression ratio engines.

detonation = very bad, explosion of the chamber gasses before the spark ignites the mixture...