LOL! I'm not a professional!
I do porn, not cars. (FYI)
Compression is the amount of pressure the combustion chambers are under.
The octane level changes the speed and cleanliness of the detonation. Pre-detonation is common on higher compression engine because the fuel will ignite easier. So higher octanes are required to guarantee a specific time for detonation AFTER the piston has reached its apex.
Read this:
Engine knocking - Wikipedia, the free encyclopedia <-- That will explain the pre-detonation and let you know why higher octanes are more important on cars with higher compressions.