All the different technical terms that get thrown around can be quite confusing to anyone who is not an experienced mechanic or a gearhead. An engine's compression ratio is used to measure the power of the motor, but before you can fully understand the compression ratio, you need to have a grasp of engine displacement.
For this we will look at the entry level 2012 Camry, which has a 2.5-liter four cylinder engine. The "2.5-liter" descriptor indicates the engine's displacement, which is the total amount of space in the cylinders. If you're comparing two four-cylinder engines and you're looking for something more powerful, you would generally want to choose the motor with the higher displacement, as this allows more room for fuel to be burned. While the difference in power is clear when comparing engines that have varying numbers of cylinders, it can be more difficult to discern with engines of the same exterior size. This is where displacement comes into play.
The displacement is the measure of the air being moved from the cylinder as the piston moves from bottom to top, also called the swept volume. However, there is a pocket of space at the top where the piston does not reach, which is unaccounted for when configuring an engine's displacement. The volume of this space is compared to the displacement of each cylinder, and is always much smaller than the cylinder's swept volume. Essentially, the more powerful an engine is, the higher its compression ratio will be. Yet, a four-cylinder engine with a 10.5:1 ratio will not outweigh a V8 with a 9.0:1 compression ratio.
When a vehicle has a higher compression ratio, it is burning more fuel at once and requires high octane fuel to function properly and get an even burn. If you use low octane fuel in an engine with a high compression ratio, you could experience engine knocking, which occurs when the fuel mixture in the combustion chamber does not burn completely during the correct time and ignites when it shouldn't. This can severely damage your engine if you're not careful. Newer models are equipped with knock sensors to adjust the timing of the combustion so drivers can fill up with lower octane fuel and not need to worry about knocks.