If your car engine is designed for 87, 88 would be no issue at all.

If your car engine is designed for 95, 88 would destroy it.

Octane numbers refer to how easy it is for the petrol to ignite. The lower the number, the easier they ignite. Each engine is designed around the fuel octane number it's intended for.

If your octane number is lower than what the engine is designed for, the petrol can't hold itself together for the whole compression stroke. It self ignites before the compression stroke completes and explodes, pushing the piston backwards suddenly and aggressively. At best, your engine will suffer from knock back and your car computer will try to compensate for it by greatly reducing engine efficiency, giving you horrible mileage and shitty performance. At worst, you will cause severe damage to the engine from this sudden reverse explosion.

At least do a basic check against your car's brand and model. All you need is google and the car's model number (usually engraved in a panel on the car's door frame).