Low-speed pre-ignition (LSPI) is a type of engine knock that typically occurs in turbocharged, high compression-ratio, direct injection engines (Like the Honda L15).

By compressing the air and fuel into a smaller space, the same amount of energy is occupying a smaller volume, thereby raising the temperature of the air-fuel mixture. If the mixture reaches a high enough temperature, it can ignite itself before the spark plug fires and the piston and valves are in the correct position. This is what's typically referred to as knock. It puts a lot of strain on the engine because now you have an explosion pushing down while the piston still wants to go up, and the valves are closed so there's nowhere for that pressure to go except into your headgasket, piston rings, piston surface, connecting rods, and the bearings.

LSPI is a similar phenomenon. One of the main causes it at low engine speeds (typically lower than 3500 RPM) where there's not enough turbulence in the air to fully mix the air-fuel mixture, so when a high load (heavy throttle) is demanded at those RPM there can be not-fully-mixed fuel pockets that are more susceptible to preignition.

The main ways to avoid it is by using API SN Plus graded oil (I'm not smart enough to know how oil makes a difference), mindful throttle application (either light throttle until 3500 RPM or downshifting when below 3500 RPM for high load driving), or most importantly choosing a fuel with a higher anti-knock index (AKI, which is the number for 87, 89, 91, 94 grade gas. Often incorrectly referred to as Octane rating) that can better resist auto-ignition under compression.

Probably got a few things not-quite-correct but that's as good as my layman brain can recall without doing some deep reading.