A bore is the inside of a car's engine where the magic happens (kind of like a playground for cars!). It's like a tunnel that's really important because it houses the pistons which are like the superheroes that move up and down to create power that moves the car (sort of like how you move your legs to ride your bike).
The bore is measured by a very special tool that checks how wide and deep the tunnel is. The wider and deeper it is, the more powerful the engine can be! When mechanics talk about an engine that's "stroked", they mean that they made the tunnels bigger.
So, a bigger bore is like a bigger playground for the car's superheroes (pistons), which means the car can have more power - but making it too big can be a bad idea because it can weaken other parts of the engine! It's important to make sure everything is balanced and working together, a bit like how you need your feet, hands and eyes all working together to ride your bike safely.